tiny pdumper fix

[Charles A. Roelli]

I'd like to push this change to the pdumper branch to allow building
on older macOS (less than 10.11, I think), where only MAP_ANON is
defined:

--- a/src/pdumper.c
+++ b/src/pdumper.c
@@ -4101,6 +4101,11 @@ dump_anonymous_allocate_w32 (void *base,
 #endif
 }
 
+/* Old versions of macOS only define MAP_ANON, not MAP_ANONYMOUS.  */
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
 static void *
 dump_anonymous_allocate_posix (void *base,
                                size_t size,

Is it well placed here, or does it belong in the header file instead?