Bug 414946 (part 2): Enable jemalloc on Mac (r=pavlov)

configure.in

@@ -2053,6 +2053,7 @@ case "$target" in
     else
         MOZ_OPTIMIZE_FLAGS="-O3 -fno-omit-frame-pointer"
     fi
+    MOZ_MEMORY=1
     _PEDANTIC=
     CFLAGS="$CFLAGS -fno-common"
     CXXFLAGS="$CXXFLAGS -fno-common"