Use a single CMake variable instead of two

CMakeLists.txt

Makefile

source/App/BitstreamExtractorApp/CMakeLists.txt

source/App/DecoderAnalyserApp/CMakeLists.txt

source/App/DecoderApp/CMakeLists.txt

source/App/EncoderApp/CMakeLists.txt

source/App/SEIFilmGrainApp/CMakeLists.txt

source/App/SEIRemovalApp/CMakeLists.txt

source/App/StreamMergeApp/CMakeLists.txt

source/App/SubpicMergeApp/CMakeLists.txt

source/Lib/CommonAnalyserLib/CMakeLists.txt

source/Lib/CommonLib/CMakeLists.txt

source/Lib/DecoderAnalyserLib/CMakeLists.txt

source/Lib/DecoderLib/CMakeLists.txt

source/Lib/EncoderLib/CMakeLists.txt

source/Lib/Utilities/CMakeLists.txt