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/cpu-intel.c b/src/crypto/cpu-intel.c
index 431e1e1..f2e0c4c 100644
--- a/src/crypto/cpu-intel.c
+++ b/src/crypto/cpu-intel.c
@@ -69,10 +69,10 @@
 #include <string.h>
 
 #if defined(OPENSSL_WINDOWS)
-#pragma warning(push, 3)
+OPENSSL_MSVC_PRAGMA(warning(push, 3))
 #include <immintrin.h>
 #include <intrin.h>
-#pragma warning(pop)
+OPENSSL_MSVC_PRAGMA(warning(pop))
 #endif
 
 #include "internal.h"