Update some obsoletes m4 macros (!41) · Merge requests · Xfce / xfce4-dev-tools · GitLab

They are obsolete since 2.69, and 2.70 now warn about it
Bump minimal autoconf to 2.69 (available since 2012)
AC_HEADER_STDC is not needed anymore: https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Particular-Headers.html