Conditionalise math.i for PHP

PHP already provides all the wrapped constants and all the wrapped
functions except fabs() (PHP provides abs() instead).  Rewrapping
the constants causes warnings or errors (depending on PHP version)
and the rewrapped functions seem to be hidden by the built-in
versions, so only wrap fabs() for PHP.

(Even a wrapper for fabs() seems of little use since abs() is already
provided, but really math.i seems of little use more generally since
any general purpose programming language will provide its own maths
functions and constants - the key motivation here is to eliminate
warnings and errors from running the testsuite.)

Examples/test-suite/li_math.i

@@ -1,7 +1,2 @@
 %module li_math
-#ifdef SWIGPHP
-// PHP already provides these functions with the same names, so just kill that
-// warning.
-%warnfilter(SWIGWARN_PARSE_KEYWORD);
-#endif
 %include math.i