[java-interop] Fix include directory specification

[jonpryor]

Commit b6431ac broke the build of src/java-interop:

Using task Exec from Microsoft.Build.Tasks.Exec, Microsoft.Build.Tasks.v4.0, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Executing: gcc -g -shared -std=c99 -o "obj/libjava-interop-m32.dylib" -m32 -DDEBUG -DJI_DLL_EXPORT -DMONODEVELOP -DMONO_DLL_EXPORT -fvisibility=hidden -Wl,-undefined -Wl,suppress -Wl,-flat_namespace -L /Library/Frameworks/Mono.framework/Libraries -lmonosgen-2.0   jni.c java-interop.c java-interop-mono.c java-interop-gc-bridge-mono.c
Environment variables being passed to the tool:
jni.c:7:10: fatal error: 'jni.h' file not found
#include <jni.h>
         ^
1 error generated.
In file included from java-interop-mono.c:1:
./java-interop-mono.h:30:11: fatal error: 'mono/metadata/assembly.h' file not found
        #include <mono/metadata/assembly.h>
                 ^
1 error generated.

The problem? There's no gcc -Ipath option, so the compiler doesn't
know where to find <jni.h> or <mono/metadata/assembly.h>.

The -I values are derived from the @(JdkIncludePath) and @(MonoIncludePath)` MSBuild item groups:

<!-- via bin/BuildDebug/JdkInfo.props -->
<PropertyGroup>
  <JdkJvmPath Condition=" '$(JdkJvmPath)' == '' ">/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/server/libjvm.dylib</JdkJvmPath>
</PropertyGroup>
<ItemGroup>
  <JdkIncludePath Condition=" '$(JdkJvmPath)' == '' " Include="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/include" />
  <JdkIncludePath Condition=" '$(JdkJvmPath)' == '' " Include="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/include/darwin" />
</ItemGroup>

<!-- via bin/BuildDebug/MonoInfo.props -->
<PropertyGroup>
  <MonoFrameworkPath Condition=" '$(MonoFrameworkPath)' == '' ">/Library/Frameworks/Mono.framework/Libraries/libmonosgen-2.0.1.dylib</MonoFrameworkPath>
</PropertyGroup>
<ItemGroup>
  <MonoIncludePath Condition=" '$(MonoFrameworkPath)' == '' " Include="/Library/Frameworks/Mono.framework/Headers/mono-2.0" />
</ItemGroup>

The intent to commit b6431ac is that if e.g. $(JdkJvmPath) is
overridden, then we shouldn't automatically generate the
@(JdkIncludePath) items, because they'll be for the "wrong" JDK.
If $(JdkJvmPath) is overridden, then @(JdkIncludePath) should be
as well. (Ditto $(MonoFrameworkPath) and @(MonoIncludePath).)

The problem is evaluation time:

During the evaluation phase of a build, imported files are
incorporated into the build in the order in which they appear.
Properties and items are defined in three passes in the following
order:

• Properties are defined and modified in the order in which they appear.
• Item definitions are defined and modified in the order in which they appear.
• Items are defined and modified in the order in which they appear.

Properties are always evaluated before items, meaning when it came
time to evaluate //JdkIncludePath/@Condition, $(JdkJvmPath) was
always defined, and thus @(JdkIncludePath) was always empty.

Which is why there are no instances of gcc -I in the gcc compile
line, and why those headers can't be found.

The fix is to instead use the <Choose/> element to ensure that the
default items are specified when the "controlling" property isn't
overridden:

<!-- via bin/BuildDebug/JdkInfo.props -->
<Choose>
  <When Condition=" '$(JdkJvmPath)' == '' ">
    <PropertyGroup>
      <JdkJvmPath>/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/server/libjvm.dylib</JdkJvmPath>
    </PropertyGroup>
    <ItemGroup>
      <JdkIncludePath Include="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/include" />
      <JdkIncludePath Include="/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/include/darwin" />
    </ItemGroup>
  </When>
</Choose>

<!-- via bin/BuildDebug/MonoInfo.props -->
<Choose>
  <When Condition=" '$(MonoFrameworkPath)' == '' ">
    <PropertyGroup>
      <MonoFrameworkPath>/Library/Frameworks/Mono.framework/Libraries/libmonosgen-2.0.1.dylib</MonoFrameworkPath>
      <MonoLibs         >-L /Library/Frameworks/Mono.framework/Libraries -lmonosgen-2.0</MonoLibs>
    </PropertyGroup>
    <ItemGroup>
      <MonoIncludePath Include="/Library/Frameworks/Mono.framework/Headers/mono-2.0" />
    </ItemGroup>
  </When>
</Choose>

This allows @(JdkIncludePath) and @(MonoIncludePath) to have
values when $(JdkJvmPath) and $(MonoFrameworkPath) aren't
overridden, which in turn allows gcc -I to be used, and allows
src/java-interop to build.