external/boringssl: Sync to 171b54.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/8ca0b4127da11d766067ea6ec4122017ba0edb0e..171b5403ee767fa0f3aecd377867db6533c3eb8f

This also updates the build file to build as C99, so BoringSSL can use
variables in for loops.

Change-Id: I48ae985fd1bed244f7ed327aefc9a13e5b17b185
diff --git a/src/crypto/test/file_test.h b/src/crypto/test/file_test.h
index e90cc86..a859127 100644
--- a/src/crypto/test/file_test.h
+++ b/src/crypto/test/file_test.h
@@ -20,19 +20,15 @@
 #include <stdint.h>
 #include <stdio.h>
 
-#if defined(_MSC_VER)
-#pragma warning(push)
-#pragma warning(disable: 4702)
-#endif
+OPENSSL_MSVC_PRAGMA(warning(push))
+OPENSSL_MSVC_PRAGMA(warning(disable: 4702))
 
 #include <string>
 #include <map>
 #include <set>
 #include <vector>
 
-#if defined(_MSC_VER)
-#pragma warning(pop)
-#endif
+OPENSSL_MSVC_PRAGMA(warning(pop))
 
 // File-based test framework.
 //