build: Restore installation of preloadable_libintl.so (regression 2021-06-15).
* gettext-tools/configure.ac (USE_INCLUDED_LIBINTL, BUILD_INCLUDED_LIBINTL,
PRELOADABLE_LIBINTL): Remove unused conditionals.
* gettext-runtime/configure.ac (PRELOADABLE_LIBINTL): Define as before in
gettext-tools/configure.ac.
* PACKAGING: Update regarding preloadable_libintl.so.
