Append new C flags when adding defines in base CMakeLists.txt.
Merge pull request #9562 from wefiends/upstream2-v3

AudioEngine-win32.ccp: Switch OpenAL include paths based on OPENAL_PLAINS_INCLUDES.
Merge pull request #9561 from wefiends/upstream-v3

CMake - Append new C flags when adding defines instead of overriding.