Build with clang-cl standalone.

Our build logic needed to revised and and clang implements more warnings
than MSVC, so GTest needed more fixes.

Bug: 200
Change-Id: I84c5dd0c51079dd9c990e08dbea7f9022a7d6842
Reviewed-on: https://boringssl-review.googlesource.com/21204
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/include/openssl/base.h b/include/openssl/base.h
index aa65b4a..adb5047 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -180,7 +180,7 @@
 #endif  // defined(BORINGSSL_SHARED_LIBRARY)
 
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 // MinGW has two different printf implementations. Ensure the format macro
 // matches the selected implementation. See
 // https://sourceforge.net/p/mingw-w64/wiki2/gnu%20printf/.