cmake: Get FindFreetype to look for versioned libraries

Since FindFreetype.cmake already extracts the version of freetype
from its header files, I have no idea why they're not automatically
using that info to look for the proper library name in lieu of
hardcoding a single version.
cmake: Get FindPNG to find the correct version of libpng

FindPNG.cmake parses libpng's version from png.h. As such, there's no
reason not to use that information when looking for the library itself.