If mman.h doesn't define MAP_NORESERVE, just define it to be 0

The MAP_NORESERVE option probably doesn't even do anything on any
operating system we run on, except for the Solaris-ish systems.

This allows the lisp kernel to compile on FreeBSD 11 (which no
longer defines MAP_NORESERVE).

lisp-kernel/memory.c

@@ -36,6 +36,14 @@
 
 #ifndef WINDOWS
 #include <sys/mman.h>
+
+/*
+ * FreeBSD 11.0, at least, doesn't define this any more. It has never
+ * actually implemented it.
+ */
+#ifndef MAP_NORESERVE
+#define MAP_NORESERVE 0
+#endif
 #endif
 
 #define DEBUG_MEMORY 0