fix compilation error on MSVC (facebook#5458)

Summary:
"__attribute__((__weak__))" was introduced in port\jemalloc_helper.h. It's not supported by Microsoft VS 2015, resulting in compile error. This fix adds a #if branch to work around the compile issue.
Pull Request resolved: facebook#5458

Differential Revision: D15827285

fbshipit-source-id: 8c5f7ad31de1ac677bd96f16c4450767de834beb

port/jemalloc_helper.h

@@ -16,6 +16,14 @@
 #define JEMALLOC_CXX_THROW
 #endif
 
+#if defined(OS_WIN) && defined(_MSC_VER)
+
+// MSVC does not have weak symbol support. As long as ROCKSDB_JEMALLOC is defined,
+// Jemalloc memory allocator is used.
+static inline bool HasJemalloc() { return true; }
+
+#else
+
 // Declare non-standard jemalloc APIs as weak symbols. We can null-check these
 // symbols to detect whether jemalloc is linked with the binary.
 extern "C" void* mallocx(size_t, int) __attribute__((__weak__));
@@ -50,4 +58,6 @@ static inline bool HasJemalloc() {
          malloc_stats_print != nullptr && malloc_usable_size != nullptr;
 }
 
+#endif
+
 #endif  // ROCKSDB_JEMALLOC