OS X compatibility fixes

CMakeLists.txt

cmake/Modules/FindGettextLib.cmake

src/CMakeLists.txt

src/cguittfont/irrUString.h

src/jthread/jevent.h

src/jthread/jsemaphore.h

src/jthread/pthread/jevent.cpp

src/jthread/pthread/jsemaphore.cpp

src/porting.cpp

src/porting.h