Improve FindFreetype.cmake for Mac
Unify TinyXML2 usage.

* should be included as simple "tinyxml2.h", but on some files was "tinyxml2/tinyxml2.h", replace this.
* Add cmake/Modules/FindTinyXML2.cmake
* As tinyxml2 for now has not prebuilt, instead sources included, add their as subdirectory if USE_PREBUILT_LIBS