Do not set number of jobs if already specified

SConstruct

@@ -9,10 +9,13 @@ import methods
 import multiprocessing
 
 # Enable aggresive compile mode if building on a multi core box
-if ARGUMENTS.get('spawn_jobs', 'yes') == 'yes':
+# only is we have not set the number of jobs already or we do
+# not want it
+if ARGUMENTS.get('spawn_jobs', 'yes') == 'yes' and \
+	int(GetOption('num_jobs')) <= 1:
 	NUM_JOBS = multiprocessing.cpu_count()
 	if NUM_JOBS > 1:
-		SetOption('num_jobs', multiprocessing.cpu_count()+1)
+		SetOption('num_jobs', NUM_JOBS+1)
 
 methods.update_version()