Adds support for generating Xcode files with CMake for iOS builds

Example, to build both Release and Debug libraries for all iOS architectures:

cd Build
cmake -GXcode -DIOS_PLATFORM=OS -DFREETYPE_LIBRARY=../Dependencies/iOS/\$\(CONFIGURATION\)/lib/libfreetype.a -DFREETYPE_INCLUDE_DIRS=../Dependencies/iOS/\$\(CONFIGURATION\)-iphoneos/include/freetype2
xcodebuild -config Debug
xcodebuild -config Release

To build for the simulator, use -DIOS_PLATFORM=SIMULATOR instead of OS