[MacPorts] #39019: create an elisp portgroup to simplify installing modules for emacs/emacs-app

[MacPorts]

#39019: create an elisp portgroup to simplify installing modules for emacs/emacs-
app
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 Reporter:  dports@…     |      Owner:  dports@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.1.3
 Keywords:               |       Port:
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 We've long had an issue with ports that install emacs modules, because
 they need to have a dependency on emacs, and usually use the emacs binary
 to byte-compile elisp code. The problem is doing this in a way that
 supports emacs-app as well. (Path dependencies aren't useful, because
 emacs and emacs-app intentionally don't install files into the same
 location.)

 Some ports have been dealing with this using a +emacs_app variant, which
 isn't an ideal solution. Several others don't have a solution at all,
 which is even worse.

 The gnuplot port checks at installtime which version is installed, and
 uses that. The attached portgroup tries to factor out the common code for
 that test so that we can use it in other ports. The assumption here is
 that all of these emacsen install into the same site-lisp directory (which
 is true of our emacs and emacs-app ports) and that they are the same major
 version so that bytecode will be compatible, so it doesn't really matter
 which one we build with.

 The portgroup just sets up some variables for use by portfiles. I didn't
 try to set up any of the build environment directly in the portgroup; the
 build systems for these modules are so different that it's not clear
 there's any common settings. Also attached an example of how to modify the
 org-mode portfile to use the portgroup.

 Any comments are appreciated!

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Ticket URL: <https://trac.macports.org/ticket/39019>
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