Apple defines MAP_ANON, not MAP_ANONYMOUS

Summary:
- Linux considers MAP_ANON deprecated, but it's all that Apple defines.
Manually set up a mapping of one to the other.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: mwang@fb.com

FB internal diff: D998511

folly/test/AtomicHashArrayTest.cpp

@@ -26,6 +26,10 @@
 #include "folly/Memory.h"
 #include <gtest/gtest.h>
 
+#if !defined(MAP_ANONYMOUS) && defined(MAP_ANON)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
 using namespace std;
 using namespace folly;