Pre-emptively fix audio/kwave before CMake 3.11.

The newer CMake respects CMAKE_REQUIRED_LIBRARIES in places that older
CMake code doesn't expect. Temporarily reset to empty for that. No
effect on CMake 3.10, which didn't use that variable in CHECK_INCLUDE_FILES
anyway.

Reported by: antoine (via exp-run)
Approved by: tcberner (mentor, implicit)