assume getopt/getopt_long is available

There is no longer a good reason to detect whether or not getopt_long()
is available. All UNIX implementations we're likely to run on have it. And
if we ever find one that doesn't the right thing to do is not fallback to
getopt() but to include the getopt_long() source in our package like we
do with the pcre2 library. Since it's licensed under LGPL we can legally
do so if it becomes necessary.

This partially addresses issue #2790.

src/fallback.cpp

@@ -1207,20 +1207,6 @@ int killpg(int pgr, int sig)
 }
 #endif
 
-#ifndef HAVE_WORKING_GETOPT_LONG
-
-int getopt_long(int argc,
-                char * const argv[],
-                const char *optstring,
-                const struct option *longopts,
-                int *longindex)
-{
-    return getopt(argc, argv, optstring);
-}
-
-
-#endif
-
 #ifndef HAVE_BACKTRACE
 int backtrace(void **buffer, int size)
 {