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/bio/file.c

@@ -88,7 +88,7 @@
 #define BIO_FP_APPEND 0x08
 
 static FILE *open_file(const char *filename, const char *mode) {
-#if defined(_WIN32) && defined(CP_UTF8)
+#if defined(OPENSSL_WINDOWS) && defined(CP_UTF8)
   int sz, len_0 = (int)strlen(filename) + 1;
   DWORD flags;
 

crypto/err/err.c

@@ -633,7 +633,7 @@ void ERR_put_error(int library, int reason, const char *function,
   }
 
   if (library == ERR_LIB_SYS && reason == 0) {
-#if defined(WIN32)
+#if defined(OPENSSL_WINDOWS)
     reason = GetLastError();
 #else
     reason = errno;

include/openssl/base.h

@@ -98,7 +98,7 @@ extern "C" {
 #define OPENSSL_APPLE
 #endif
 
-#if defined(WIN32) || defined(_WIN32)
+#if defined(_WIN32)
 #define OPENSSL_WINDOWS
 #endif