Fix conflict between different uses of GETTEXTLIBDIR.

Fix conflict between different uses of GETTEXTLIBDIR.

Rationale: The programs installed in $(prefix)/lib/gettext/ are at different
locations before installation: some are in the source tree, some are in the
build tree.

* gettext-tools/src/msginit.c (project_id, project_id_version): Test
GETTEXTLIBDIR_SRCDIR, not GETTEXTLIBDIR.
(plural_forms): Test GETTEXTLIBDIR_BUILDDIR, not GETTEXTLIBDIR.
* gettext-tools/tests/msginit-3: Set GETTEXTLIBDIR_SRCDIR, not GETTEXTLIBDIR.
* gettext-tools/tests/msginit-4: Set both GETTEXTLIBDIR_SRCDIR and
GETTEXTLIBDIR_BUILDDIR.