Fix MinGW build breakage


git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4282 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/config.h.cmake.in b/config.h.cmake.in
index eee0f77..6b1ca3d 100644
--- a/config.h.cmake.in
+++ b/config.h.cmake.in
@@ -6,6 +6,6 @@
 #cmakedefine HAVE_VSNPRINTF
 
 /* MS Visual Studio 2008 and newer doesn't know ssize_t */
-#if defined(HAVE_GNUTLS) && defined(WIN32)
-#define ssize_t long
+#if defined(HAVE_GNUTLS) && defined(WIN32) && !defined(__MINGW32__)
+typedef size_t ssize_t;
 #endif