Use underscored I/O function names for WINAPI_FAMILY.

Suggested by E. Timothy Uy.

gzguts.h

@@ -35,6 +35,13 @@
 #  include <io.h>
 #endif
 
+#ifdef WINAPI_FAMILY
+#  define open _open
+#  define read _read
+#  define write _write
+#  define close _close
+#endif
+
 #ifdef NO_DEFLATE       /* for compatibility with old definition */
 #  define NO_GZCOMPRESS
 #endif