Stricter check for math.h functions

GCC is sometimes able to optimise constant calls to these functions,
incorrectly indicating that they exist.  Unoptimised calls will then
fail to link.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@21749 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -675,7 +675,8 @@ check_mathfunc(){
     disable $func
     check_ld "$@" <<EOF && enable $func
 #include <math.h>
-int main(void){ $func(0); return 0; }
+float foo(float f) { return $func(f); }
+int main(void){ return 0; }
 EOF
 }