Remove unnecessary #undefs in Port header.

The Port function names are actually camel-cased and
as such do not collide with the math macros, and undefining
the names in the header is just annoying on platforms where
you want to use the system macros in the implementation.

dmd2/root/port.h

@@ -41,14 +41,6 @@ struct Port
     static double dbl_min;
     static longdouble ldbl_max;
 
-#if !defined __HAIKU__ || __OpenBSD__
-#elif __GNUC__
-    // These conflict with macros in math.h, should rename them
-    #undef isnan
-    #undef isfinite
-    #undef isinfinity
-    #undef signbit
-#endif
     static int isNan(double);
     static int isNan(longdouble);