Don't use .def files for Meson/Visual Studio builds

Hi,

This attempts to improve the Visual Studio bits for the recently-added Meson build files for FontConfig, namely:

* Use CMake to look for FreeType too, since it is likely that FreeType was built using CMake for Visual Studio builds, which do not generate pkg-config files for us but instead generates CMake config files. The default dependency search wouldn't work for CMake builds of FreeType because the package name is different, freetype rather than freetype2, and the versioning scheme is different as well (please see FreeType's doc/VERSION.txt).

• Instead of using .def files, just define FcPublic accordingly when building with Visual Studio-style compilers, so we have less maintainenance burden, as FcPublic decorations are already in place. (issue #245 (closed)--sorry, I don't know the autotools build system well enough in the Visual Studio regard to say anything about that, or whether we should drop the fontconfig.def.in template).

With blessings, thank you!