[tlbuild] building one engine

[Karl Berry]

Mojca sent me this message a couple years ago. Unfortunately it is still
completely relevant. I figured I might as well reply to the list for the
archives, at least; nothing actionable, except for someone who wants to
delve deep into the TL autosystem.

    Date: Mon, 1 May 2017 09:57:07 +0200
    From: Mojca Miklavec <mojca.miklavec.lists at gmail.com>

    I did a checkout of *luatex* svn repo and I tried
        ../source/configure  --disable-all-pkgs --enable-luatex
        make
    but "make" doesn't do anything.
    
    And if I go to "web2c/luatexdir", then it fails building
    ../../../../source/texk/web2c/lib/openclose.c:12:29: error:
    ptexenc/ptexenc.h: No such file or directory

It's necessary to both cd to the web2c build dir and then make the
specific target. Also, it's immaterial whether --enable-luatex is given
to configure (or Build, whichever):

./configure --disable-all-pkgs  # or ./Build
cd Work/texk/web2c
make luatex

This works (I hope) because all the dependencies are (should be) there
for make regardless of being enabled, including (re-)running
configure. It worked for me in the TL repository just now. Also, this is
what we do in the pdftex source repository, which is a cut-down TL like
(I assume) luatex.

I've updated tlbuild.texi with a new section "Build one engine" with the
above.

I'm not completely sure, but I believe the underlying issue is that the
engines are not "package"s in the configure system peb set up; "web2c"
is the only such package. Therefore enable/disable don't work as
expected with the engines, or the (c)web and other tools under web2c/.

Futhermore, there is an extra problem with ptexenc (hence the
compilation error above), in that it is one of the special "texk"
libraries -- the other being kpathsea. So, unfortunately, there is
currently no way to --disable it. Doing that, with the implication that
its dependencies are also disabled, is something I want to do, but have
not yet managed.

Such is the once and current reality :(. --thanks, karl.