getopt.h: Add optional_argument macro.

This allows compiling code that uses this macro on Windows.
(See next commit for details).

src/getopt.h

@@ -6,6 +6,7 @@
 /* macros defined by this include file */
 #define no_argument       0
 #define required_argument 1
+#define optional_argument 2
 #define OPTIONAL_ARG      2
 
 /* types defined by this include file */