ignore explicit #warnings in compiler tests

Originally committed as revision 5872 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -214,7 +214,7 @@ add_extralibs(){
 }
 
 check_cmd(){
-    "$@" 2>&1 | tee -a $logfile | { ! grep -q .; }
+    "$@" 2>&1 | tee -a $logfile | { ! grep -E -q -v '#warning|^In file'; }
 }
 
 check_cc(){