clover: Force gcc and g++ to fix clang builds.
glsl: For my stupidness remove $(VISIBILITY_CFLAGS).

.libs/st_glsl_to_tgsi.o: In function `ir_if':
/home/abuild/rpmbuild/BUILD/Mesa/build/src/mesa/../../../src/glsl/ir.h:796: undefined reference to `vtable for ir_if'
.libs/st_glsl_to_tgsi.o: In function `ir_loop_jump':
/home/abuild/rpmbuild/BUILD/Mesa/build/src/mesa/../../../src/glsl/ir.h:1439: undefined reference to `vtable for ir_loop_jump'
collect2: error: ld returned 1 exit status

A fix must go in "mesa: Build libmesa shared." patch.