correctly detect MINGW

[oschonrock]

Nice lib, thanks!

compiling as is on mingw fails on finding endian.h because this file does not exist on MINGW.
I propose to widen the _MSC_VER detection at top of sha1.cpp, etc....

as below:

$ git diff
diff --git a/sha1.cpp b/sha1.cpp
index 8331e92..6a4a658 100644
--- a/sha1.cpp
+++ b/sha1.cpp
@@ -7,7 +7,7 @@
 #include "sha1.h"

 // big endian architectures need #define __BYTE_ORDER __BIG_ENDIAN
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) && !defined(__MINGW32__) && !defined(__MINGW64__)
 #include <endian.h>
 #endif