Description:
------------
In latest php versions it checks for isinf with

AC_CHECK_DECLS([isnan, isinf], [], [], [[#include <math.h>]]), 

which leads to defining 

#define HAVE_DECL_ISINF 0

when isinf is not declared.

Later it checks for it with 

#ifdef HAVE_DECL_ISINF
#define zend_isinf(a) isinf(a)
#elif defined(INFINITY)
/* Might not work, but is required by ISO C99 */
#define zend_isinf(a) (((a)==INFINITY || (a)==-INFINITY)?1:0)
#elif defined(HAVE_FPCLASS)
#define zend_isinf(a) ((fpclass(a) == FP_PINF) || (fpclass(a) == FP_NINF))
#else
#define zend_isinf(a) 0
#endif

This should be changed to something like 

#if defined(HAVE_DECL_ISINF) && (HAVE_DECL_ISINF!=0) 
...


This is also true for HAVE_DECL_ISNAN and HAVE_DECL_ISFINITE

Otherwise linking php fails with 'Undefined symbol isinf...'.