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.

Originally committed as revision 21749 to svn://svn.ffmpeg.org/ffmpeg/trunk

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
 }