Salix OS

It is not present in the repositories.

http://mirrors.xmission.com/salix/slkbuild/14.2/
http://mirrors.xmission.com/salix/sbo/14.2/

>st (0.8.1)
st is a simple terminal implementation for X.

> youtube-dl (2018.04.09)
youtube-dl is a small command-line program to download videos
from YouTube.com and a few more sites. It requires the Python
interpreter, version 2.6, 2.7, or 3.2+, and it is not platform
specific. It should work on your Unix box, on Windows or on
Mac OS X. It is released to the public domain, which means you
can modify it, redistribute it or use it however you like.

Please ...

The slkbuild repos for 14.2 are empty but both software are already available in:
http://mirrors.xmission.com/salix/sbo/14.2/
and also in:
http://slackware.uk/salix/sbo/14.2/

So I don't understand your request.

EDIT: Although the tarballs be there, slapt-src and sourcery don't find them. I don't know why.

Because they are already there as packages. Available with slapt-get/gslapt/spi.

I should have remembered that...

wiki,

slapt-srcrc

Also edit your /etc/slapt-get/slapt-srcrc file to point to 14.2 instead of 14.1:

BUILDDIR=/usr/src/slapt-src
PKGEXT=txz
SOURCE=http://slackware.uk/salix/slkbuild/14.2/
SOURCE=http://slackware.uk/salix/sbo/sbo/14.2/

Yes, but I can not edit the "config.def.h"

Configuration.
Configuration is done with config.h. Read the comments in the generated config.h to edit it according to your needs. Defaults are stored in config.def.h.

And mpv + youtube-dl fails.
MPV Error?

The mpv package probably is not built with that functionality, but I dare say you could easily enough rebuild it to provide it.

I don't understand about configuration for slapt-src. Then editing slapt-srcrc should be all that is needed - except that you probably shouldn't even do that because

a) that is not how to install the packages you want

b) you shouldn't change the source in the config file if you need specific things from elsewhere