fix incorrect use of PLATFORM_DEFAULT with cmake

CMakeLists.txt

@@ -153,7 +153,7 @@ if(UNIX AND NOT APPLE)
 else()
 	set(PLATFORM_DEFAULT OFF)
 endif()
-option(WITH_OPENMP        "Enable OpenMP (has to be supported by the compiler)" PLATFORM_DEFAULT)
+option(WITH_OPENMP        "Enable OpenMP (has to be supported by the compiler)" ${PLATFORM_DEFAULT})
 unset(PLATFORM_DEFAULT)
 
 
@@ -199,7 +199,7 @@ if(MINGW)
 else()
 	set(PLATFORM_DEFAULT OFF)
 endif()
-option(WITH_CODEC_FFMPEG        "Enable FFMPeg Support (http://ffmpeg.org)" PLATFORM_DEFAULT)
+option(WITH_CODEC_FFMPEG        "Enable FFMPeg Support (http://ffmpeg.org)" ${PLATFORM_DEFAULT})
 unset(PLATFORM_DEFAULT)
 
 option(WITH_CODEC_SNDFILE       "Enable libsndfile Support (http://www.mega-nerd.com/libsndfile)" OFF)