rework CMakeLists, fix zlib/minizip includes

 * rename project to OpenTechBFG
 * remove irrxml, freetype and oggvorbis stuff, as the project doesn't seem to use it at all
 * fix dynamic linking of libraries as it's issued here https://github.com/OpenTechEngine/OpenTechBFG/issues/7 (tested on Mint 17 64bit; minizip still does some trouble though)
 * do some cleanup in CMakeLists.txt (I could revert some of it if it's too much)