[running TL program from build tree]

[Karl Berry]

Let me reply separately to this:

    [ Aside: What's the right way to run TeX tools from their build
    tree? If I just attempt to run xdvi-bin naively, ....
    
    warning: kpathsea: configuration file texmf.cnf not found in these
    directories: ...

Unfortunately, it is necessary to set both TEXMFCNF and TEXMFROOT.
For example:

$ pwd
/home/karl/Build/source/Work/texk/xdvik

$ env TEXMFROOT=/usr/local/texlive/2021 \
      TEXMFCNF=/usr/local/texlive/2021/texmf-dist/web2c \
      ./xdvi-bin /tmp/t2a.dvi
[files are found]

    The quick and dirty workaround was to copy xdvi-bin out of the build
    tree to /Library/TeX/texbin/ and run it from there. ]

I often do that too. --happy debugging, karl.