build: Avoid xgettext crash on MSYS2

build: Avoid xgettext crash on MSYS2

isatty() on MSYS2 returns non-zero if the fd is stdout and is redirected
to /dev/null.  That lets xgettext to produce colorized output and leads
to an assertion failure during terminal type detection.

Although the problem should be fixed in MSYS2, isatty() could behave
wrongly in many ways on Windows.  Since gtk+ doesn't need colorized
output, it would be safer to bypass the terminal dependent code.

See also:
https://lists.gnu.org/archive/html/bug-gettext/2015-04/msg00004.html

https://bugzilla.gnome.org/show_bug.cgi?id=748346