MSVC: fix compile errors due to macro redefinitions

Skip errno.h definitions if they are already defined.

Signed-off-by: Karsten Blees <blees@dcon.de>
Acked-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -32,7 +32,9 @@ typedef int socklen_t;
 #define WEXITSTATUS(x) ((x) & 0xff)
 #define WTERMSIG(x) SIGTERM
 
+#ifndef EWOULDBLOCK
 #define EWOULDBLOCK EAGAIN
+#endif
 #define SHUT_WR SD_SEND
 
 #define SIGHUP 1
@@ -46,8 +48,12 @@ typedef int socklen_t;
 #define F_SETFD 2
 #define FD_CLOEXEC 0x1
 
+#ifndef EAFNOSUPPORT
 #define EAFNOSUPPORT WSAEAFNOSUPPORT
+#endif
+#ifndef ECONNABORTED
 #define ECONNABORTED WSAECONNABORTED
+#endif
 
 struct passwd {
 	char *pw_name;