Cycles: Change OpenCL split kernel to use single program by default

Single program builds twice as fast as multi programs, so its better for
users to have it as the default.

intern/cycles/util/util_debug.cpp

@@ -118,7 +118,7 @@ void DebugFlags::OpenCL::reset()
 	}
 	/* Initialize other flags from environment variables. */
 	debug = (getenv("CYCLES_OPENCL_DEBUG") != NULL);
-	single_program = (getenv("CYCLES_OPENCL_SINGLE_PROGRAM") != NULL);
+	single_program = (getenv("CYCLES_OPENCL_MULTI_PROGRAM") == NULL);
 }
 
 DebugFlags::DebugFlags()