--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -193,19 +193,16 @@ getenv(const char* name)
   return nullptr;
 }
 #endif
 
 #ifndef XP_WIN
 #ifndef MADV_FREE
 #define MADV_FREE MADV_DONTNEED
 #endif
-#ifndef MAP_NOSYNC
-#define MAP_NOSYNC 0
-#endif
 #endif
 
 
 // Some tools, such as /dev/dsp wrappers, LD_PRELOAD libraries that
 // happen to override mmap() and call dlsym() from their overridden
 // mmap(). The problem is that dlsym() calls malloc(), and this ends
 // up in a dead lock in jemalloc.
 // On these systems, we prefer to directly use the system call.