detection of isinf, isnan and other math functions

Hello All,

I review recent changes done by  Stefan Kost into configure script I 
note that isinf and isnan are to detected properly.

Functions isinf and isnan are definitions not real functions, so correct 
macro has to be :
AC_CHECK_DECLS([isinf, isnan], [], [], [[#include <math.h>]])

As ouher side I could not found where code use HAVE_ISNAN, HAVE_ISINF, 
HAVE_POW, HAVE_FLOOR and HAVE_FABS are used.
May be good patch is to remove all those checks.

Regards,
Roumen