Fix WtFindPango on recent Debian/Ubuntu/derivatives

Recent versions of Debian (and thus, Ubuntu) install freetype.h to /usr/include/freetype2/freetype.h
    
This change makes CMake find freetype.h, while still maintaining compatibility with systems which install it under e. g.
      /usr/include/freetype2/freetype/freetype.h, or
      /usr/include/freetype/freetype.h