Log Message:
chat/ejabberd: Recover from breakage due to untested erlang update

A recent update to erlang broke building of ejabberd.  The problem is
essentially "languages should have their own packaging system"
disease.  ejabberd includes rebar and rebar3 as erlang binaries
("beam"), and the new erlang refused to run them.  Therefore, remove
them and symlink.

Also, two source files had constructs objected to by the new compiler.

rebar objects to https_proxy being syntactically invalid, even though
I think it isn't trying to download anything.  It's not really wrong
to object, so this seems like a pkgsrc infrastructure bug.  Work
around it by not setting https_proxy to fail.

While investigating, clean up multiple things, likely accumulated over time:

  - explicitly ask for rebar to build, rather than relying on defaults
  - drop dependency on elixir; configure defaults to off and it isn't used
  - stop disabling the erlang version check; the build succeeds without it
  - drop USE_GLOBAL_DEPS from the environment because that string
    doesn't appear in the sources.  Probably it is a previous method
    of asking for reasonable behavior (rather than fetching and
    building dependencies).
  - remove, but leave commented, another workaround to avoid building
    dependencies
  - add patch comments