Avoid using |WIN32| and use |OPENSSL_WINDOWS| instead.

MSVC and clang-cl automatically define |_WIN32| but |WIN32| is only
defined if a Windows header file has been included or if -DWIN32 was
passed on the command line. Thus, it is always better to test |_WIN32|
than |WIN32|. The convention in BoringSSL is to test |OPENSSL_WINDOWS|
instead, except for the place where |OPENSSL_WINDOWS| is defined.

Change-Id: Icf3e03958895be32efe800e689d5ed6a2fed215f
Reviewed-on: https://boringssl-review.googlesource.com/5553
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/err/err.c

diff --git a/crypto/err/err.c b/crypto/err/err.c
index 417d322..24824e8 100644
--- a/crypto/err/err.c
+++ b/crypto/err/err.c
@@ -633,7 +633,7 @@
   }
 
   if (library == ERR_LIB_SYS && reason == 0) {
-#if defined(WIN32)
+#if defined(OPENSSL_WINDOWS)
     reason = GetLastError();
 #else
     reason = errno;