Fix check_func_headers. The reference to the function under test was being

optimized out by gcc, thus rendering the test useless.
This new test throws a warning in mingw-w64, but that should not be a problem.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@19440 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -646,8 +646,7 @@ check_func_headers(){
     check_ld "$@" <<EOF && enable $func && enable_safe $headers
 $incs
 int main(int argc, char **argv){
-    (void) $func;
-    return 0;
+    return (long) $func;
 }
 EOF
 }