[vcpkg] Clean up CMake build system

[strega-nil]

There are quite a few changes to the CMake build system packaged up into
one set here:

• Added toolsrc/cmake/utilities.cmake, which contains the following:
  • vcpkg_detect_compiler -- get the name of the C++ compiler, as one
    of {gcc, clang, msvc}
  • vcpkg_detect_standard_library -- get the name of the standard
    library we're linking to, as one of {libstdc++, libc++, msvc-stl}
  • vcpkg_detect_std_filesystem -- figure out how to link and call
    into C++17's filesystem; whether one needs to link to stdc++fs or
    c++fs, and whether to use <filesystem> or
    <experimental/filesystem>.
• Added a VCPKG_WARNINGS_AS_ERRORS, split off from
  VCPKG_DEVELOPMENT_WARNINGS, which allows one to use the development
  warnings without passing -Werror
• Rename DEFINE_DISABLE_METRICS to VCPKG_DISABLE_METRICS -- the
  former will now print a deprecation message and set the latter.
• Now, print a deprecation message on WERROR; it doesn't do anything
  since the behavior it requested is now the default.
• Pass -std=c++17 if the compiler allows it, instead of -std=c++1z
• Do some code movement
• Pass USE_STD_FILESYSTEM if possible, instead of only on minGW
  • Renamed to VCPKG_USE_STD_FILESYSTEM

Additionally, we now pass /W4 in Debug mode on x86 in the Visual
Studio build system; this brings it in line with the CMake build system,
and the x64 Visual Studio build system.

And finally, we make some minor code changes to support compiling in
VCPKG_DEVELOPMENT_WARNINGS mode.

[BillyONeal]

Thanks!

[strega-nil]

Thanks for the review billy!

[miketimofeev]

@strega-nil @BillyONeal Hi, folks!
It looks like this PR breaks bootstrap process. There is an error both on macOS and ubuntu:

Extracting ninja...
Extracting ninja... done.
CMake Error: The source directory "/home/testAdm2/toolsrc" does not exist.

Previous commit works fine

[strega-nil]

@miketimofeev that's really odd... I think I know why it's broken though, lemme fix it. I guess we should test whether it's possible to bootstrap from not-the-vcpkg directory.

[miketimofeev]

@strega-nil thank you!