Don't mess with GPU stuff if we don't have them.

main.c

@@ -5344,6 +5344,7 @@ int main (int argc, char *argv[])
 	} else
 		chosen_kernel = KL_NONE;
 
+#ifdef HAVE_OPENCL
 	gpu_threads = nDevs * opt_g_threads;
 	if (total_devices) {
 		if (total_devices > nDevs)
@@ -5366,6 +5367,9 @@ int main (int argc, char *argv[])
 			gpu_devices[i] = true;
 		total_devices = nDevs;
 	}
+#else
+  gpu_threads = 0;
+#endif
 
 	if (!gpu_threads && !forced_n_threads) {
 		/* Maybe they turned GPU off; restore default CPU threads. */