configure: simplify check_lib function

Originally committed as revision 23777 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -731,11 +731,7 @@ check_lib(){
     header="$1"
     func="$2"
     shift 2
-    temp_extralibs "$@"
-    check_header $header && check_func $func && add_extralibs "$@"
-    err=$?
-    restore_flags
-    return $err
+    check_header $header && check_func $func "$@" && add_extralibs "$@"
 }
 
 check_lib2(){