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>

configure.ac

@@ -1475,6 +1475,10 @@ if test "x$enable_opencl" = xyes; then
         AC_MSG_ERROR([cannot enable OpenCL without Gallium])
     fi
 
+    if test "x$enable_gallium_llvm" != xyes; then
+        AC_MSG_ERROR([cannot enable OpenCL without LLVM])
+    fi
+
     if test $GCC_VERSION_MAJOR -lt 4 -o $GCC_VERSION_MAJOR -eq 4 -a $GCC_VERSION_MINOR -lt 7; then
         AC_MSG_ERROR([gcc >= 4.7 is required to build clover])
     fi