bug#29097: 25.1; to support '_' as a word constituent, subword-mode regexes need updating

[Eli Zaretskii]

> From: Chris Hecker <checker@d6.com>
> Date: Tue, 31 Oct 2017 20:51:23 -0700
> Cc: yamato@redhat.com
> 
> If you like using identifiers with underscores (_) in them, like
> C macros or even just regular identifiers, you can add _ as a word
> constituent in the syntax tables, but if you're using subword-mode
> then the forward and backwards regexes need updating to work
> correctly.  Here is the relevant section of my .emacs c-mode hook:

Perhaps we should make subword-forward-regexp and
subword-backward-regexp defcustoms?