After verifying from git[1] I noticed that this bug[2] is still not fixed.

[1] http://git.xfce.org/xfce/xfconf/tree/po/POTFILES.in
[2] http://bugs.gentoo.org/show_bug.cgi?id=444996

Unfortunately this is fatal instead of plain warning:

Making check in po
make[1]: Entering directory `/var/tmp/portage/xfce-base/xfconf-4.10.0/work/xfconf-4.10.0/po'
XGETTEXT_ARGS="--keyword=Q_ --from-code=UTF-8" INTLTOOL_EXTRACT="/usr/bin/intltool-extract" XGETTEXT="/usr/bin/xgettext --keyword=Q_ --from-code=UTF-8" srcdir=. /usr/bin/intltool-update --gettext-package xfconf --pot
rm -f missing notexist
srcdir=. /usr/bin/intltool-update -m
The following files contain translations and are currently not in use. Please
consider adding these to the POTFILES.in file, located in the po/ directory.

xfconf-perl/xs/Xfconf.c
xfconf-perl/xs/XfconfBinding.c
xfconf-perl/xs/XfconfChannel.c

If some of these files are left out on purpose then please add them to
POTFILES.skip instead of POTFILES.in. A file 'missing' containing this list
of left out files has been written in the current directory.
Please report to http://bugzilla.xfce.org/
if [ -r missing -o -r notexist ]; then \
  exit 1; \
fi
make[1]: *** [check] Error 1

The most simplest solution I can think of, if these files don't have any translatable strings is to add file po/POTFILES.skip with content of:

xfconf-perl/xs/Xfconf.c
xfconf-perl/xs/XfconfBinding.c
xfconf-perl/xs/XfconfChannel.c

Thank you!