fix: only use -Werror=enum-conversion in testing when CC supports it

Despite this option existing in the GCC documentation [0], it does not seem to
actually be implemented in any packaged version of GCC. It has been implemented
in GCC [1], but seemingly only for a yet-to-ship version.

  [0]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
  [1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78736