r300g: set a dummy vertex buffer in context_create

so that the driver doesn't crash if an app doesn't set any vertex buffers.
radeon, clover: Make it compile on LLVM >= r171366.
gallium/svga: Make sure -std=gnu99 is set.
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.
Suppress clang's warnings about unused CFLAGS and CXXFLAGS.
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.
i915: Conditionally build an i915g driver instead of two i915 drivers.