Enable more warnings & treat warnings as errors on Windows.

Change-Id: I2bf0144aaa8b670ff00b8e8dfe36bd4d237b9a8a
Reviewed-on: https://boringssl-review.googlesource.com/3140
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/mem.c b/crypto/mem.c
index 11ef335..a8a8c08 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -65,7 +65,9 @@
 #include <string.h>
 
 #if defined(OPENSSL_WINDOWS)
+#pragma warning(push, 3)
 #include <Windows.h>
+#pragma warning(pop)
 #else
 #include <strings.h>
 #endif