Set needed defines for UINT64_C in gcm_test.cc.

Change-Id: Ia38acd73e18a78b6bf9b9d10339f920b7f105c85
Reviewed-on: https://boringssl-review.googlesource.com/12601
Reviewed-by: Adam Langley <agl@google.com>

crypto/modes/gcm_test.cc

@@ -46,6 +46,13 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  * ==================================================================== */
 
+/* Per C99, various stdint.h and inttypes.h macros (the latter used by
+ * internal.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
+
 #include <stdio.h>
 #include <string.h>