Fix jemalloc folly weak symbol issue

Summary:
Fix jemalloc folly weak symbol issue

(Note: this ignores all push blocking failures!)

Reviewed By: davidtgoldblatt

Differential Revision: D22420581

fbshipit-source-id: 136fa6a8c74d8fea3fc96ba1d205266f2f2fa02c

folly/memory/detail/MallocImpl.cpp

@@ -33,6 +33,7 @@ const char* mallctlnametomibWeak = nullptr;
 const char* mallctlbymibWeak = nullptr;
 const char* MallocExtension_Internal_GetNumericPropertyWeak = nullptr;
 #elif !FOLLY_HAVE_WEAK_SYMBOLS
+#if (!defined(USE_JEMALLOC) && !defined(FOLLY_USE_JEMALLOC)) || FOLLY_SANITIZE
 void* (*mallocx)(size_t, int) = nullptr;
 void* (*rallocx)(void*, size_t, int) = nullptr;
 size_t (*xallocx)(void*, size_t, size_t, int) = nullptr;
@@ -44,6 +45,7 @@ int (*mallctl)(const char*, void*, size_t*, void*, size_t) = nullptr;
 int (*mallctlnametomib)(const char*, size_t*, size_t*) = nullptr;
 int (*mallctlbymib)(const size_t*, size_t, void*, size_t*, void*, size_t) =
     nullptr;
+#endif
 bool (*MallocExtension_Internal_GetNumericProperty)(
     const char*,
     size_t,

folly/memory/detail/MallocImpl.h

@@ -42,6 +42,9 @@ bool MallocExtension_Internal_GetNumericProperty(const char*, size_t, size_t*)
     __attribute__((__weak__));
 #endif
 #else
+#if (!defined(USE_JEMALLOC) && !defined(FOLLY_USE_JEMALLOC)) || FOLLY_SANITIZE
+// we do not want to declare these if we have jemalloc support
+// to avoid redefinition errors
 extern void* (*mallocx)(size_t, int);
 extern void* (*rallocx)(void*, size_t, int);
 extern size_t (*xallocx)(void*, size_t, size_t, int);
@@ -53,6 +56,7 @@ extern int (*mallctl)(const char*, void*, size_t*, void*, size_t);
 extern int (*mallctlnametomib)(const char*, size_t*, size_t*);
 extern int (
     *mallctlbymib)(const size_t*, size_t, void*, size_t*, void*, size_t);
+#endif
 extern bool (
     *MallocExtension_Internal_GetNumericProperty)(const char*, size_t, size_t*);
 #ifdef _MSC_VER