Clean up build files

Hi,

This will attempt to clean up and the build files on Windows a bit, by:

• Using dependency: ... only for FontConfig, FreeType and Cairo (meaning Visual Studio users should use Cairo 1.17.x or later; there were numerous improvements for Cairo on Windows in 1.17.x or later), so no more cc.find_library() and cc.has_header() business for these in the build files.
• Clean up generating the pkg-config files as a result, except for FreeType, which may have been found by Meson via CMake.
• Make builds using subprojects for deps easier, meaning we set default options for people that need or wish to build Pango with FontConfig support in the deps as needed.
• Use HarfBuzz's DirectWrite support also in the unlikely case where one does not enable Cairo support.
• Update the Windows build documentation accordingly.

With blessings, thank you!