I'm not able to compile with gettext 0.17 since it's not available on my
Debian (I have only 0.18).
Anyway, you should NOT compile with autotools unless you have a good reason to
do that (autotools may be removed from WeeChat in future).
Cmake is the preferred way to compile weechat, and it should work with any
version of gettext.
If you really can't use cmake (which is better and faster than autotools),
then I guess you'll have to patch the configure.ac manually.
Or if you have a better solution that would work on a machine with only
gettext 0.18, please tell me :)