Require source files define __STDC_FORMAT_MACROS to use BN FMT macros.

inttypes.h kindly requires a feature macro in C++ on some platforms, due
to a bizarre footnote in C99 (see footnote 191 in section 7.8.1). As
bn.h is a public header, we must leak this wart to the consumer. On
platforms with unfriendly inttypes.h headers, using BN_DEC_FMT1 and
friends now require the feature macro be defined externally.

This broke the Chromium Android Clang builder:
http://build.chromium.org/p/chromium.linux/builders/Android%20Clang%20Builder%20%28dbg%29/builds/59288

Change-Id: I88275a6788c7babd0eae32cae86f115bfa93a591
Reviewed-on: https://boringssl-review.googlesource.com/4688
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/bn_test.cc

@@ -67,6 +67,11 @@
  * Sheueling Chang Shantz and Douglas Stebila of Sun Microsystems
  * Laboratories. */
 
+/* For BIGNUM format macros. */
+#if !defined(__STDC_FORMAT_MACROS)
+#define __STDC_FORMAT_MACROS
+#endif
+
 #include <stdio.h>
 #include <string.h>
 
@@ -76,7 +81,6 @@
 #include <openssl/mem.h>
 
 #include "../crypto/test/scoped_types.h"
-#include "internal.h"
 
 
 static const int num0 = 100; // number of tests

crypto/bn/internal.h

@@ -125,12 +125,6 @@
 
 #include <openssl/base.h>
 
-/* Some versions of inttypes.h will not define print macros in C++ unless
- * __STDC_FORMAT_MACROS is set. */
-#if !defined(__STDC_FORMAT_MACROS)
-#define __STDC_FORMAT_MACROS
-#endif
-
 #if defined(OPENSSL_X86_64) && defined(_MSC_VER) && _MSC_VER >= 1400
 #pragma warning(push, 3)
 #include <intrin.h>

include/openssl/bn.h

@@ -139,7 +139,12 @@ extern "C" {
  * will allow you to work with numbers until you run out of memory. */
 
 
-/* BN_ULONG is the native word size when working with big integers. */
+/* BN_ULONG is the native word size when working with big integers.
+ *
+ * Note: on some platforms, inttypes.h does not define print format macros in
+ * C++ unless |__STDC_FORMAT_MACROS| defined. As this is a public header, bn.h
+ * does not define |__STDC_FORMAT_MACROS| itself. C++ source files which use the
+ * FMT macros must define it externally. */
 #if defined(OPENSSL_64_BIT)
 #define BN_ULONG uint64_t
 #define BN_BITS2 64