In Slackware64 14, I still got the error message when running xxgdb:
xxgdb: all ptys in use
It seems even with the patch of xxgdb-1.12-debian-pty.patch.xz in the stock build scripts, it does not work in Slackware.
I developed a separate patch which use the posix calls. With the this patch and xxgdb-1.12-debian-pty.patch.xz removed, I got a working xxgdb.
The patch could be downloaded here: xxgdb-pty.patch.xz.