Suppress clang's warnings about unused CFLAGS and CXXFLAGS.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
configure.ac: Save user {C,CXX}FLAGS and append them at end.

This way the user has the privilege of last decision and so the option to build an optimized debug build again.
radeon: Build libllvmradeon the right way.

Better build system integration for:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=91a160b
http://cgit.freedesktop.org/mesa/mesa/commit/?id=5b2855b
gallium/targets: Make use of prebuilt libdricommon.la.
Drop support for --enable-static / --disable-shared.
gallium/auxiliary: Build libgallium shared.
Drop last parts of compatibility for the old Mesa build system.
mapi: Build libglapi always shared.
mesa: Build libmesa shared.
Install all internal shared libs to $(libdir)/mesa-$VERSION.
Makefile.am: s:-no-undefined:-Wl,--no-undefined to make it work.
gallium/drivers: Build libs -shared.
glx: Get rid of libglx.la.
gbm: Get rid of libgbm_dri.la.
i915: Conditionally build an i915g driver instead of two i915 drivers.
freedreno: Make print_sequence a macro to fix clang.
freedreno: One Makefile.am with a Makefile.sources is enough.
clover: Force gcc and g++ to fix clang builds.