Fix build warning if _SCL_SECURE_NO_WARNINGS is defined globally

Thanks to shohei.yoshida@dena.jp for reporting the issue and providing
a patch.

Bug: 302
Change-Id: I1200a917ef4b791822712feafece19cb21988d55
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/38684
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/tool/transport_common.cc b/tool/transport_common.cc
index d04cb7d..7c5e962 100644
--- a/tool/transport_common.cc
+++ b/tool/transport_common.cc
@@ -16,7 +16,9 @@
 // pointer, on grounds that MSVC cannot check them. Unfortunately, there is no
 // way to suppress the warning just on one line. The warning is flagged inside
 // the STL itself, so suppressing at the |std::copy| call does not work.
+#if !defined(_SCL_SECURE_NO_WARNINGS)
 #define _SCL_SECURE_NO_WARNINGS
+#endif
 
 #include <openssl/base.h>