bug#39678: warn about stray backslashes

[Paul Eggert]

On 5/23/22 13:03, Paul Eggert wrote:
> it makes sense to warn about EREs like '(*)', '(*a)', '(+)', '(+a)', 
> '({1})', '({1}a)' as POSIX does not specify their behavior, their 
> semantics are unpredictable with GNU grep, and it's plausible that 
> people are making mistakes in this area.

I installed the attached to do that. As before, most of the changes were 
in Gnulib's dfa module.

With all these changes, we now see behavior like this:

$ echo a | src/grep -oi '\a'
src/grep: warning: stray \ before a

which is not ideal (so I'll leave the bug report open), but at least 
this gives the user a warning that the pattern is not on the up-and-up.

0001-build-update-gnulib-submodule-to-latest.patch
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0002-grep-warn-about-x-etc.patch
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