Define __STDC_CONSTANT_MACROS in bn_test.cc.

C99 decided that, like PRI* macros, UINT64_C and friends should be
conditioned on __STDC_CONSTANT_MACROS in C++. C++11 then decided this
was ridiculous and overruled this decision. However, Android's headers
in older NDKs mistakenly followed the C99 rules for C++, so work around
this.

This fixes the android_arm bots.

Change-Id: I3b49e8dfc20190ebfa78876909bd0dccd3e210ea
Reviewed-on: https://boringssl-review.googlesource.com/11089
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/bn/bn_test.cc

@@ -67,7 +67,12 @@
  * Sheueling Chang Shantz and Douglas Stebila of Sun Microsystems
  * Laboratories. */
 
-/* For BIGNUM format macros. */
+/* Per C99, various stdint.h and inttypes.h macros (the latter used by bn.h) are
+ * unavailable in C++ unless some macros are defined. C++11 overruled this
+ * decision, but older Android NDKs still require it. */
+#if !defined(__STDC_CONSTANT_MACROS)
+#define __STDC_CONSTANT_MACROS
+#endif
 #if !defined(__STDC_FORMAT_MACROS)
 #define __STDC_FORMAT_MACROS
 #endif