Fix build failures due to missing JEMALLOC_CXX_THROW macro (facebook#…

…5053)

Summary:
JEMALLOC_CXX_THROW is not defined for earlier versions of jemalloc (e.g. 3.6), causing builds to fail on some platforms. Fixing it. Closes facebook#4869
Pull Request resolved: facebook#5053

Differential Revision: D14390034

Pulled By: sagar0

fbshipit-source-id: b2b7a03cd377201ef385eb521f65bae85c558055

port/jemalloc_helper.h

@@ -12,6 +12,10 @@
 #include <jemalloc/jemalloc.h>
 #endif
 
+#ifndef JEMALLOC_CXX_THROW
+#define JEMALLOC_CXX_THROW
+#endif
+
 // 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__));