Added some platform-dependent preprocessor directives

AVQt/CMakeLists.txt

AVQt/cmake/Config.cmake.in

AVQt/include/AVQt/AVQt.h

AVQt/src/capture/DesktopCaptureFactory.cpp

AVQt/src/communication/Message.cpp

AVQt/src/communication/PacketPadParams.cpp

AVQt/src/decoder/VideoDecoderFactory.cpp

AVQt/src/encoder/VideoEncoderFactory.cpp

AVQt/src/renderers/FallbackFrameMapper.cpp

AVQt/src/renderers/OpenGLFrameMapperFactory.cpp

CMakeLists.txt

LibScreenCapture

Player/CMakeLists.txt

Player/main.cpp