Fix configure tests for -Wfoo option support.

Drooped support for -Wunused-macros and -Wno-unused-macros.  The
warnings serve no useful purpose, and causing subsequent configure
checks for -Wfoo flags to get messed up.

Split each test of -Wfoo into a separate check. Include -Werror in
the individual flag tests to ensure the test compile fails if the
other flag isn't supported.  Without this, clang issues a diagnostic
message about the flag being ignored, but otherwise carries on, fooling
configure into thinking the flag is in fact supported.