build: Install gettext.h with the LGPL license header, not with the GPL header.
Reported by Joachim Metz <joachim.metz@gmail.com>
in <https://lists.gnu.org/archive/html/bug-gettext/2012-09/msg00000.html>.
* gettext-tools/gnulib-lib/Makefile.am (gettextsrcdir, gettextsrc_DATA): Remove
variables.
* gnulib-local/Makefile.am (gettextsrcdir, gettextsrc_DATA): Define them here.
