build: conditionally use -Wempty-body

-Wempty-body is not available on all gcc versions but we were using it
unconditionally. Also remove the usage from the clang case. clang still
defines `__GNUC__` so it still gets all the gcc specific flags.

This should fix the build on systems with older gcc versions like OpenBSD which
still comes bundled with gcc 4.2 for license issues.