gtk: Hardcode LC_CTYPE as C.utf-8

gtk: Hardcode LC_CTYPE as C.utf-8

Commit 2cb5d2a4 removed setlocale() for everything except LC_MESSAGES in
order to avoid unwanted side effects such as using the wrong decimal
separator in generated JSON objects. However, the problem that unsetting
LC_CTYPE caused is that non-ASCII characters are considered
non-printable now and therefore the GTK menus display question marks for
accented letters, Chinese characters etc.

A first attempt to fix this [1] was rejected because even just setting
LC_CTYPE to the user's locale (and thereby modifying the semantics of
the ctype.h functions) could have unwanted effects that we're not aware
of yet.

Recently, however, glibc introduced a new locale "C.utf-8" that just
uses UTF-8 as its charset, but otherwise leaves the semantics alone.
Just setting the right character set is enough for our use case, so we
can just hardcode this one without having to be afraid of nasty side
effects.

Older systems that don't have the new locale will continue displaying
question marks, but this should fix the problem for most users.

[1] https://lists.gnu.org/archive/html/qemu-devel/2015-12/msg03591.html
    ('Re: gtk: use setlocale() for LC_MESSAGES only')

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20170131100945.8189-1-kwolf@redhat.com

[ kraxel: change C.utf-8 to C.UTF-8 ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>