MacOS doesn't have malloc_usable_size()

Summary:
Use malloc_size() instead

Test Plan: HHVM build on MacOS

Reviewed By: joelm@fb.com

Subscribers: trunkagent, folly-diffs@, yfeldblum

FB internal diff: D1875364

Signature: t1:1875364:1425061479:7d8cec757c186fa8c30b6500fca6b2fe4c1ed137

folly/Portability.h

@@ -31,6 +31,10 @@
 
 #include <folly/CPortability.h>
 
+#ifdef __APPLE__
+# include <malloc/malloc.h>
+#endif
+
 #if FOLLY_HAVE_SCHED_H
  #include <sched.h>
  #ifndef FOLLY_HAVE_PTHREAD_YIELD
@@ -255,4 +259,11 @@ using namespace FOLLY_GFLAGS_NAMESPACE;
 #include <TargetConditionals.h>
 #endif
 
+// MacOS doesn't have malloc_usable_size()
+#if defined(__APPLE__) && !defined(FOLLY_HAVE_MALLOC_USABLE_SIZE)
+inline size_t malloc_usable_size(void* ptr) {
+  return malloc_size(ptr);
+}
+#endif
+
 #endif // FOLLY_PORTABILITY_H_