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
Merge pull request #54 from pgquiles/debianfreetype

Fix WtFindPango on recent Debian/Ubuntu/derivatives