Win32: minimized redefinition of ssize_t.

All variants of GCC have ssize_t available, there is no need to
redefine it.
diff --git a/src/os/win32/ngx_win32_config.h b/src/os/win32/ngx_win32_config.h
index 2cb8e19..ecdf6ad 100644
--- a/src/os/win32/ngx_win32_config.h
+++ b/src/os/win32/ngx_win32_config.h
@@ -185,7 +185,7 @@
 #endif
 
 
-#ifndef __MINGW64_VERSION_MAJOR
+#ifndef __GNUC__
 typedef int                 ssize_t;
 #endif