[cmake] support previous RTAUDIO_BUILD_* variables

[cmake] support previous RTAUDIO_BUILD_* variables

Compromise for changes in CMake build introduced by previous commit
that would interfere with use with add_subdirectory.

Re-adds:

- RTAUDIO_BUILD_TESTING
- RTAUDIO_BUILD_SHARED_LIBS
- RTAUDIO_BUILD_STATIC_LIBS

These now override BUILD_TESTING, BUILD_SHARED_LIBS, BUILD_STATIC_LIBS,
but only if they are defined and are not "".