Define ssize_t on mswin build

prism/util/pm_buffer.h

@@ -15,6 +15,14 @@
 #include <stdlib.h>
 #include <string.h>
 
+#if defined(_MSC_VER) && !defined(ssize_t)
+# ifdef _WIN64
+#   define ssize_t __int64
+# else
+#   define ssize_t int
+# endif
+#endif
+
 /**
  * A pm_buffer_t is a simple memory buffer that stores data in a contiguous
  * block of memory.