__STDC_VERSION__ is not defined for c89 compilers

Signed-off-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NRich Salz <rsalz@openssl.org>

MR: #1522

include/openssl/e_os2.h

@@ -321,8 +321,10 @@ typedef unsigned __int64 uint64_t;
  * some systems (e.g. Mac OS X).
  */
 # ifndef PRIu64
-#  if (__STDC_VERSION__ >= 199901L)
-#   include <inttypes.h>
+#  ifdef __STDC_VERSION__
+#   if (__STDC_VERSION__ >= 199901L)
+#    include <inttypes.h>
+#   endif
 #  endif
 #  ifndef PRIu64
 #   define PRIu64 "lu"