configure: error out when building opencl without LLVM

Cc: Tom Stellard <thomas.stellard@amd.com>
Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
1 file changed