configure: Test GCC-specific compiler flags only when using GCC

Commit 23373d01d "configure: Test compiler flags" (v4.4.0) made
configure test whether compiler flags work, to let us use flags that
work only for some compilers.

Unfortunately, the Sun C compiler accepts invalid options with a
warning, breaking the test.  Observed on Solaris 10.

Fix by testing the compiler flags only when AC_PROG_CC detected GCC.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>