This allows to ignore lines from the default build.config if needed.
For example to not include packages run make WITHOUT=index

When the index files are concatenated, (`index/*` in `build.config`),
their order is related to the collation locale used by the shell. By
explicitely setting this to the C locale, we make sure that the order is
consistent, no matter the user's locale.

It is especially useful in teams where users and systems have different
locales.

[ci-skip]

It'll now 'git checkout erlang.mk'.

"make up" will be how it's done from now on, rather than the
more manual method I was using until now.

I am fully aware that this command fetches the erlang.mk repo
twice. But it works, and honestly fetching the repo isn't too
expensive or long so that will do.

Option `--rfc-3339` is specific to GNU Coreutils' date(1). This change
fixes the use of `make summary` on other platforms such as FreeBSD.

The underscore is integrated to the tiem format at the same time; no
need for an additional $(subst).

Better error reporting and can now build everything in parallel.
Failed builds are kept; others are deleted.

The following command builds everything and then gives a diff
of what got fixed/broken since last time ("> " is newly broken,
"< " is newly fixed).

  make packages -j 32 -k; make summary

Also fixes two bugs with the new "no .app.src" method.
All tests now pass with both methods.

We can now test specific cases with make check c=$CASE.