[build] Allow building with msbuild.

DO NOT MERGE!!!

TODO: Need the Java.Interop bump for the generator fix!
dotnet/java-interop#46

Fix the solution and project files so that `msbuild` may be used to
build the solution instead of requiring `xbuild`.

There were a few issues that `msbuild` didn't like:

1. MSBuild doesn't like the "extra" configuration mappings in
    Xamarin.Android.sln.

2. MSBuild doesn't like the presence of `.dll` within `@(Reference)`
    entries. `<Reference Include="System.dll" />` is Bad™, so
    Don't Do That™.™.

3. Turning `$(AndroidSupportedAbis)` into an item group is...broken.

(1) and (2) are straightforward fixes. (3) requires some explanation.

`src/monodroid` needs to *only* build `libmonodroid.so` for the
non-"host" ABIs within `$(AndroidSupportedAbis)`. It needs this
restriction because non-selected ABIs may not be configured in
`$(AndroidNdkDirectory)`, and thus can't be built.

This *could* be done by following
`build-tools/mono-runtimes/mono-runtimes.projitems` and doing lots of
`Condition`s on `$(AndroidSupportedAbisForConditionalChecks)`:

	<_MonoRuntime Include="armeabi-v7a" Condition="$(AndroidSupportedAbisForConditionalChecks.Contains(':armeabi-v7a:'))" />
	...

However, that's kinda ugly when *all* we need is the ABI name, so
`monodroid.projitems` was "cute":

	<PropertyGroup>
		<_SupportedAbis>$(AndroidSupportedAbis.Replace(':', ';'))</_SupportedAbis>
	</PropertyGroup>
	<ItemGroup>
		<_MonoRuntime Include="$(_SupportedAbis)" Exclude="@(HostOSName)" />
	</ItemGroup>
	<!-- @(_MonoRuntime) is `armeabi-v7a` by default -->

This works...on xbuild, but *not* `msbuild`. Doh!

(`msbuild` is "smart" and doesn't treat the `;` as an item separator,
so if `$(AndroidSupportedAbis)` is `host-Darwin;armeabi-v7a` then
MSBuild treats the `;` as part of the filename -- NOT a filename
separator -- and `@(_MonoRuntime)` contains *one* item with
an `%(Identity)` of `host-Darwin;armeabi-v7a`. On the one hand, this
is kinda awesome and answers the question "how can you have a filename
that contains `;`?", but on the other hand it broke my project!)

The only fix I could think of was to use `.Split(':')`:

	<_MonoRuntime Include="$(AndroidSupportedAbis.Split(':'))" Exclude="@(HostOSName)" />

That provides desired behavior with `msbuild`, but `xbuild` doesn't
support it and appears to either *ignore* it, or treat it literally.

Other attempts to fix things so that `msbuild` behaves as desired
would cause `xbuild` to break; they appear to be
*mutually incompatible* here.

To resolve this, add a new `<ValueToItems/>` task to
`Xamarin.Android.Tools.BootstrapTasks.dll`. This new task takes a
value, splits on a given separator, and turns it into an
`<ItemGroup/>`, allowing particular items to be excluded:

	<ValueToItems
	    Value="$(AndroidSupportedAbis)"
	    Split=":"
	    Exclude="@(HostOSName)">
	  <Output TaskParameter="Items" ItemName="_MonoRuntime" />
	</ValueToItems>

This allows for `@(_MonoRuntime)` to have the desired value on both
`msbuild` and `xbuild`, allowing `src/monodroid` to build as desired.

Xamarin.Android.sln

@@ -81,8 +81,6 @@ Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|AnyCPU = Debug|AnyCPU
 		Release|AnyCPU = Release|AnyCPU
-		XAIntegrationDebug|Any CPU = XAIntegrationDebug|Any CPU
-		XAIntegrationRelease|Any CPU = XAIntegrationRelease|Any CPU
 		XAIntegrationDebug|AnyCPU = XAIntegrationDebug|AnyCPU
 		XAIntegrationRelease|AnyCPU = XAIntegrationRelease|AnyCPU
 	EndGlobalSection

src/Mono.Android.Export/Mono.Android.Export.csproj

@@ -36,15 +36,15 @@
       <HintPath>$(OutputPath)..\v1.0\mscorlib.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.dll">
+    <Reference Include="System">
       <HintPath>$(OutputPath)..\v1.0\System.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Core.dll">
+    <Reference Include="System.Core">
       <HintPath>$(OutputPath)..\v1.0\System.Core.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Xml.dll">
+    <Reference Include="System.Xml">
       <HintPath>$(OutputPath)..\v1.0\System.Xml.dll</HintPath>
       <Private>False</Private>
     </Reference>

src/Mono.Android/Mono.Android.csproj

@@ -39,23 +39,23 @@
       <HintPath>$(OutputPath)..\v1.0\mscorlib.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.dll">
+    <Reference Include="System">
       <HintPath>$(OutputPath)..\v1.0\System.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Core.dll">
+    <Reference Include="System.Core">
       <HintPath>$(OutputPath)..\v1.0\System.Core.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Net.Http.dll">
+    <Reference Include="System.Net.Http">
       <HintPath>$(OutputPath)..\v1.0\System.Net.Http.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Runtime.Serialization.dll">
+    <Reference Include="System.Runtime.Serialization">
       <HintPath>$(OutputPath)..\v1.0\System.Runtime.Serialization.dll</HintPath>
       <Private>False</Private>
     </Reference>
-    <Reference Include="System.Xml.dll">
+    <Reference Include="System.Xml">
       <HintPath>$(OutputPath)..\v1.0\System.Xml.dll</HintPath>
       <Private>False</Private>
     </Reference>

src/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks.csproj

@@ -39,6 +39,7 @@
     <Compile Include="Xamarin.Android.Tools.BootstrapTasks\GenerateProfile.cs" />
     <Compile Include="Xamarin.Android.Tools.BootstrapTasks\GetNugetPackageBasePath.cs" />
     <Compile Include="Xamarin.Android.Tools.BootstrapTasks\Which.cs" />
+    <Compile Include="Xamarin.Android.Tools.BootstrapTasks\ValueToItems.cs" />
   </ItemGroup>
   <ItemGroup>
     <Content Include="Xamarin.Android.Tools.BootstrapTasks.targets">

src/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks/ValueToItems.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Xamarin.Android.Tools.BootstrapTasks
+{
+	public class ValueToItems : Task
+	{
+		[Required]
+		public  string              Value           { get; set; }
+
+		public  string              Split           { get; set; }
+
+		public  ITaskItem[]         Exclude         { get; set; }
+
+		[Output]
+		public  ITaskItem[]         Items           { get; set; }
+
+		[Output]
+		public ITaskItem Location { get; set; }
+
+		public override bool Execute ()
+		{
+			Log.LogMessage (MessageImportance.Low, $"Task {nameof (ValueToItems)}");
+			Log.LogMessage (MessageImportance.Low, $"  {nameof (Value)}: {Value}");
+			Log.LogMessage (MessageImportance.Low, $"  {nameof (Split)}: '{Split}'");
+			Log.LogMessage (MessageImportance.Low, $"  {nameof (Exclude)}:'");
+			foreach (var e in (Exclude ?? new ITaskItem[0])) {
+				Log.LogMessage (MessageImportance.Low, $"    {e}");
+			}
+
+			var items       = new List<ITaskItem> ();
+			var excludes    = Exclude?.Select (e => e.ItemSpec)?.ToList ();
+
+			var values      = Value.Split (new [] { Split }, StringSplitOptions.RemoveEmptyEntries);
+			foreach (var v in values) {
+				if (excludes != null && excludes.Any (e => string.Equals (e, v, StringComparison.OrdinalIgnoreCase)))
+					continue;
+				items.Add (new TaskItem (v));
+			}
+
+			Items   = items.ToArray ();
+
+			Log.LogMessage (MessageImportance.Low, $"  [Output] {nameof (Items)}:");
+			foreach (var e in Items) {
+				Log.LogMessage (MessageImportance.Low, $"    {e}");
+			}
+
+			return !Log.HasLoggedErrors;
+		}
+	}
+}
+

src/monodroid/monodroid.projitems

@@ -1,11 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <PropertyGroup>
-    <_SupportedAbis>$(AndroidSupportedAbis.Replace(':', ';'))</_SupportedAbis>
-  </PropertyGroup>
-  <ItemGroup>
-    <_MonoRuntime Include="$(_SupportedAbis)" Exclude="@(HostOSName)" />
-  </ItemGroup>
   <ItemGroup>
     <_RequiredProgram Include="xxd" />
   </ItemGroup>

src/monodroid/monodroid.targets

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\Xamarin.Android.Tools.BootstrapTasks.dll" TaskName="Xamarin.Android.Tools.BootstrapTasks.ValueToItems" />
   <UsingTask AssemblyFile="..\..\bin\Build$(Configuration)\Xamarin.Android.Tools.BootstrapTasks.dll" TaskName="Xamarin.Android.Tools.BootstrapTasks.Which" />
   <Import Project="monodroid.projitems" />
   <PropertyGroup>
@@ -8,7 +9,16 @@
   <ItemGroup>
     <CFiles Include="jni\**\*.c" />
   </ItemGroup>
+  <Target Name="_GetRuntimes">
+    <ValueToItems
+        Value="$(AndroidSupportedAbis)"
+        Split=":"
+        Exclude="@(HostOSName)">
+      <Output TaskParameter="Items" ItemName="_MonoRuntime" />
+    </ValueToItems>
+  </Target>
   <Target Name="_BuildRuntimes"
+      DependsOnTargets="_GetRuntimes"
       Inputs="@(CFiles);jni\Android.mk"
       Outputs="@(_MonoRuntime->'$(OutputPath)\%(Identity)\libmono-android.$(_Conf).so')">
     <Which Program="%(_RequiredProgram.Identity)" Required="True" />