fix Solaris build and maybe other missing getopt_long

git-svn-id: https://hamlib.svn.sourceforge.net/svnroot/hamlib/trunk@1991 7ae35d74-ebe9-4afe-98af-79ac388436b8

macros/gr_pwin32.m4

@@ -73,6 +73,7 @@ extern char * optarg;
 extern int optind, opterr, optopt;
 #endif
 #ifndef HAVE_GETOPT_LONG
+struct option;
 int getopt_long (int argc, char * const argv[], const char * optstring,
 			const struct option * longopts, int * longindex);
 #endif