Under some conditions autoconf incorrectly detects that gcc doesn't support -Werror

If CFLAGS or CXXFLAGS contains "-Wall" when running configure, autoconf incorrectly detects that gcc doesn't support -Werror

This is problematic because it is an important aspect of our policy that there should be no compiler warnings, and this bug stops this being enforced.

Notably, the fedora package build has this issue and has been shipping with some unnoticed warnings for a while.