Mesa from git master (head 3c3a2b51b85a90cfe670e1ca81ce86c8fa0d448d) has a
undefined symbol in r600_dri.so, which prevents it from being loaded:

$ LIBGL_DEBUG=verbose glxinfo
libGL: OpenDriver: trying /usr/lib64/dri/r600_dri.so
libGL error: dlopen /usr/lib64/dri/r600_dri.so failed
(/usr/lib64/dri/r600_dri.so: undefined symbol:
_ZTVN10__cxxabiv120__si_class_type_infoE)
libGL error: unable to load driver: r600_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: r600

configure line:
--with-dri-drivers= --with-gallium-drivers=swrast,r600 --enable-gallium-llvm 
--enable-gallium-egl --with-egl-platforms=x11,drm --enable-gles1 --enable-gles2
--enable-glx-tls --enable-texture-float --enable-shared-glapi

I tried adding radeonsi to the list and see if this fixes it, but in order to
build this I have to install LLVM 3.2, which I'm not planning to do today.