build: Install auxiliary programs in $(libexecdir), not $(libdir).
The GNU Coding Standards say that auxiliary programs should be installed in a
subdir of $(libexecdir).
<https://www.gnu.org/prep/standards/html_node/Directory-Variables.html>
Originally, I did not do this because I also wanted to follow the Linux
Filesystem Hierarchy Standard, which in version 2 did not support /usr/libexec.
This has changed in 2015: The FHS version 3 now allows installation into
/usr/libexec.
<https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s07.html>
Reported by Bastien Roucariès <rouca@debian.org> in
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1032010>.
* gettext-tools/src/Makefile.am (pkglibexecdir): New variable.
(DEFS): Pass a LIBEXECDIR macro.
(hostname_CPPFLAGS, urlget_CPPFLAGS, cldr_plurals_CPPFLAGS, hostname_LDFLAGS,
urlget_LDFLAGS, cldr_plurals_LDFLAGS, install-exec-local, installdirs-local,
uninstall-local): Use pkglibexecdir instead of pkglibdir.
* gettext-tools/src/msginit.c (project_id, project_id_version, plural_forms):
Test $GETTEXTLIBEXECDIR_SRCDIR instead of $GETTEXTLIBDIR_SRCDIR. Use LIBEXECDIR
instead of LIBDIR.
(get_user_email, language_team_address): Use LIBEXECDIR instead of LIBDIR.
* gettext-tools/tests/msginit-3: Set GETTEXTLIBEXECDIR_SRCDIR instead of
GETTEXTLIBDIR_SRCDIR.
* gettext-tools/tests/msginit-4: Likewise. Set GETTEXTLIBEXECDIR_BUILDDIR
instead of GETTEXTLIBDIR_BUILDDIR.
