configure: slightly more robust looking check for math functions.

I dont know on which system the prior check failed but an advanced
linker on some platform might have optimized func() out as unreachable.

Patch taken from http://floss.freebox.fr
Author: unknown
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -708,7 +708,7 @@ check_mathfunc(){
     check_ld "$@" <<EOF && enable $func
 #include <math.h>
 float foo(float f) { return $func(f); }
-int main(void){ return 0; }
+int main(void){ return (int) foo; }
 EOF
 }