configure: Add -Wno-typedef-redefinition to CFLAGS (for Clang)

configure: Add -Wno-typedef-redefinition to CFLAGS (for Clang)

Without the -Wno-typedef-redefinition option, clang complains if a typedef
gets redefined in gnu99 mode (since this is officially a C11 feature). This
used to also happen with older versions of GCC, but since we've bumped our
minimum GCC version to 4.8, all versions of GCC that we support do not seem
to issue this warning in gnu99 mode anymore. So this has become a common
problem for people who only test their code with GCC - they do not notice
the issue until they submit their patches and suddenly patchew or a
maintainer complains.

Now that we do not urgently need to keep the code clean from typedef
redefintions anymore with recent versions of GCC, we can ease the
situation with clang, too, and simply shut these warnings off for good.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190427154539.11336-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>