Define BORINGSSL_201510.

I've used these defines to easy the update of BoringSSL in Android
because Android's external/boringssl is a different git repository from
the rest of Android and thus it's not possible to land changes the
atomically update several things at once.

For this I tended just to add this define in the Android copy of
BoringSSL, but we're starting to see that bleed into other situations
now so it's looking like this will be generally useful.

These defines may be added when useful but shouldn't build up: once the
change has been done, the #if'ed code elsewhere that uses it should be
cleaned up. So far, that's worked ok. (I.e. we've had a BORINGSSL_201509
that correctly disappeared.)

Change-Id: I8cbb4731efe840cc798c970d37bc040b16a4a755
Reviewed-on: https://boringssl-review.googlesource.com/6442
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/base.h

@@ -108,6 +108,7 @@ extern "C" {
 #endif
 
 #define OPENSSL_IS_BORINGSSL
+#define BORINGSSL_201510
 #define OPENSSL_VERSION_NUMBER 0x10002000
 #define SSLEAY_VERSION_NUMBER OPENSSL_VERSION_NUMBER