package/x11r7/xlib_libXpm: disable gettext detection when not needed

Patch originates from FreeBSD:
https://lists.freebsd.org/pipermail/freebsd-x11/2015-July/016528.html
https://svnweb.freebsd.org/ports/head/x11/libXpm/Makefile?r1=384234&r2=391122&pathrev=391122

Fixes
http://autobuild.buildroot.net/results/f24/f24dc74c111690f068833ff6fc72df9a0853d210/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>