remove local getopt_long() implementation

getopt_long() is widely supported these days, removing it from the
source tree should be okay.

Here's documentation or the implementation of getopt_long on many
systems:

All the BSDs:
https://man.netbsd.org/NetBSD-9.2-STABLE/getopt_long.3
https://man.openbsd.org/OpenBSD-7.0/getopt_long
https://www.freebsd.org/cgi/man.cgi?query=getopt_long
https://leaf.dragonflybsd.org/cgi/web-man?command=getopt_long

Illumos (OpenSolaris fork):
https://illumos.org/man/3c/getopt_long

The musl, newlib, and uclibc-ng libcs:
https://gogs.waldemar-brodkorb.de/oss/uclibc-ng/src/master/libc/unistd/getopt_long-simple.c#L49-L52
https://git.musl-libc.org/cgit/musl/tree/src/misc/getopt_long.c#n140
https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libc/stdlib/getopt.c;h=d4f225a7a735dc741d2874eb6c131b6fce2967c5;hb=HEAD#l475

Mingw-w64:
https://github.com/mirror/mingw-w64/blob/master/mingw-w64-crt/misc/getopt.c#L542-L549

Mac OS X:
https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/getopt_long.3.html