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.