build-sys: unify configure.ac syntax
build-sys: use iconv library detections from gettext

This patch took copy of config.rpath from gettext 0.18.2 to build-aux/
directory.  Software projects usually does not need to do that, because
the file is copied by gettext autopoint in autogen.sh or bootstrap.#

In Fluxbox case calling autopoint seems wrong, as it would generate po/
directory with necessary make automation, adn 'msgid' & 'msgstr' files,
but that is not how the translations are done in this project.