configure: allow disabling warnings #5689
I'm not fluent with m4, so there might be a better or cleaner way to do that.
With `--enable-warnings`, it was not possible to disable warnings via CFLAGS that got explicitly enabled with that warnings. Now warnings are not enabled anymore if they are explicitly disabled (or enabled) in CFLAGS. This works for at least GCC, clang, and TCC as they have corresponding `-Wno-` options for every warning. Closes