Don't try to use the malloc_usable_size portability implementation if we have JEMalloc

Summary:
This is only an issue for the OSS HHVM build on OSX, where we have JEMalloc headers included directly.
In the OSS HHVM build, we define `USE_JEMALLOC` globally if we have JEMalloc, so gate the portability implementation behind that.

Reviewed By: mzlee

Differential Revision: D3237449

fb-gh-sync-id: 07cf9b31114723ffd9f2b29cf449bee0ed8f5703
fbshipit-source-id: 07cf9b31114723ffd9f2b29cf449bee0ed8f5703

folly/portability/Malloc.cpp

@@ -16,6 +16,7 @@
 
 #include <folly/portability/Malloc.h>
 
+#ifndef USE_JEMALLOC
 #if defined(__APPLE__) && !defined(FOLLY_HAVE_MALLOC_USABLE_SIZE)
 #include <malloc/malloc.h>
 
@@ -27,3 +28,4 @@ extern "C" size_t malloc_usable_size(void* addr) {
   return _msize(addr);
 }
 #endif
+#endif

folly/portability/Malloc.h

@@ -18,6 +18,11 @@
 
 #include <stdlib.h>
 
+#ifdef USE_JEMALLOC
+// JEMalloc provides it's own implementation of
+// malloc_usable_size, and that's what we should be using.
+#include <jemalloc/jemalloc.h>
+#else
 #ifndef __APPLE__
 #include <malloc.h>
 #endif
@@ -30,3 +35,4 @@ extern "C" size_t malloc_usable_size(void* ptr);
 #elif defined(_WIN32)
 extern "C" size_t malloc_usable_size(void* ptr);
 #endif
+#endif