fix build on gcc4.8.3 (aws#1156)

We recently had a team reach out to us about our broken build on gcc
4.8.3. This is due to a bug that we've previously encountered in this
commit: 6566103

We may have to add new CI to check for this, but it seems like this
specific version is hard to find. We already have CI for gcc 4.8.5.

Author: samuel40791765

tool/speed.cc

@@ -20,7 +20,12 @@
 
 #include <assert.h>
 #include <errno.h>
+
+#ifndef __STDC_FORMAT_MACROS
+#define __STDC_FORMAT_MACROS
+#endif
 #include <inttypes.h>
+
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>