build: use gettext-h

We were using the gnulib's gettext module with tricks in
bootstrap.conf to avoid useless files.  Instead, use gnulib's
gettext-h module.

* .travis.yml: Force Gettext 0.18.3 on Trusty.
* bootstrap.conf: Use gettext-h instead of gettext.
(excluded_files): Remove.
* configure.ac (AM_GNU_GETTEXT_VERSION): Bump to 0.19.

m4/.gitignore

@@ -57,7 +57,6 @@
 /iconv.m4
 /include_next.m4
 /intdiv0.m4
-/intl-thread-locale.m4
 /intl.m4
 /intldir.m4
 /intlmacosx.m4