bug#23234: unexpected results with charset handling in GNU grep 2.23

[Paul Eggert]

Paul Eggert wrote:
> Perhaps grep should work around this problem on systems like Fedora 23 where the
> underlying C library does not conform to the next version of POSIX. It sounds
> like a new gnulib module or two might do the trick. This should fix the problems
> that Björn mentions.

I've started on this by changing the mbrtowc module in gnulib to work around the 
future-POSIX incompatibility of mbrtowc in glibc. See:

http://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=b7bc3c1a4e78add4cbad39ae1a0c4fb0747b483f

I plan to change GNU grep to use this new facility, and to add some grep test 
cases for this issue.