Fix byte-compilation warnings

[d-torrance]

We move up some defvar's, call declare-function on a function from another package, and remove the use of an obsolete variable to fix some warnings that were emitted when byte-compiling.

We also add a GitHub workflow to byte-compile pull requests.

[d-torrance]

Huh, the workflow isn't running. Maybe it has to be on the main branch first?

It ran successfully when I tested it using on: push instead of on: pull_request: https://github.com/d-torrance/M2-emacs/actions/runs/10659953599/job/29543250635

[d-torrance]

I noticed a silly typo I made in the last regex in #73 after all of a sudden a bunch of my errors weren't highlighting. I pushed the fix to main and rebased this on top of it.

[mahrud]

I'm a little unsure how to review this PR. Can you think of anyone else to ask?

[d-torrance]

Maybe @DanGrayson? We're the only three folks with a halfway decent number of commits in this repository.

Anyways, here's a bit more explanation if that helps. I noticed that when I started up Emacs, I had a bunch of M2-related warnings. You can reproduce them on the command line from the current tip of the main branch:

$ emacs --batch -f batch-byte-compile M2*.el M2-symbols.el.gz
Loading /etc/emacs/site-start.d/00debian.el (source)...
Loading /etc/emacs/site-start.d/50auctex.el (source)...
Loading /usr/share/emacs/site-lisp/auctex.el (source)...
Loading /usr/share/emacs/site-lisp/preview-latex.el (source)...
Loading /etc/emacs/site-start.d/50autoconf.el (source)...
Loading /etc/emacs/site-start.d/50boxes.el (source)...
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading debian-ispell (native compiled elisp)...
Loading /var/cache/dictionaries-common/emacsen-ispell-default.el (source)...
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el (source)...
Loading /etc/emacs/site-start.d/50emms.el (source)...
Loading /etc/emacs/site-start.d/50gcl.el (source)...
Loading /etc/emacs/site-start.d/50gtk-doc-tools.el (source)...
Loading /etc/emacs/site-start.d/50latexmk.el (source)...
Loading /etc/emacs/site-start.d/50lilypond-data.el (source)...
Loading /etc/emacs/site-start.d/50mu4e.el (source)...
Loading /etc/emacs/site-start.d/50recutils.el (source)...
recutils removed but not purged, skipping setup
Loading /etc/emacs/site-start.d/50tcsh.el (source)...

In M2-comint-mode:
M2.el:55:46: Warning: reference to free variable ‘M2-error-regexp-alist’
M2.el:57:15: Warning: reference to free variable ‘M2-transform-file-match-alist’
M2.el:129:7: Warning: assignment to free variable ‘M2-common-menu’
M2.el:156:4: Warning: reference to free variable ‘M2-common-menu’

In M2-set-demo-buffer:
M2.el:472:9: Warning: assignment to free variable ‘M2-demo-buffer’

In M2-switch-to-demo-buffer:
M2.el:477:21: Warning: reference to free variable ‘M2-demo-buffer’

In M2-get-input-from-demo-buffer:
M2.el:503:32: Warning: reference to free variable ‘M2-demo-buffer’

In end of data:
M2.el:537:4: Warning: the function ‘compilation-forget-errors’ is not known to be defined.
uncompressing M2-symbols.el.gz...
uncompressing M2-symbols.el.gz...done

In toplevel form:
M2-symbols.el.gz:10:8: Warning: ‘max-specpdl-size’ is an obsolete variable (as of 29.1).

The solutions for each of these issues:

• The "free variable" warnings were coming from the fact that we weren't calling defvar on the variables before we were using them. (And for M2-common-menu, we weren't calling it at all!) Moving up the corresponding defvar statements (and adding one for M2-common-menu) fixes the warnings.
• The "function ... not known to be defined" warning was because the byte compiler didn't know that compile will be loaded and compilation-forget-errors available at runtime. Adding declare-function let's it know that it's okay.
• According to the documentation, max-specpdl-size doesn't do anything anymore, so we just remove it.

The workflow hopefully will run the above check on pull requests.

[mahrud]

At this point, I'm a little worried that if we merge this now there won't be enough time to discover any potential bugs. Let's merge this after the release?

[d-torrance]

What bugs might there be? The changes are all very minor -- basically just moving around some code and removing a call to a function that's been a noop for the last few Emacs releases.