cmake: disable default NDEBUG differently Before this patch `NDEBUG` was force-disabled, preventing a build with debug asserts disabled. After this patch `NDEBUG` works again when passed as a custom build option, e.g.: `-DCMAKE_C_FLAGS=-DNDEBUG` Previously submitted as #988, which was merged, but the commit vanished from master and ended up missing from both 3.8.3 and 3.9.0 releases.