Also bump required autoconf version to 2.64, as required by Gnulib.
Also add --conditional-dependencies to the flags. See: https://lists.gnu.org/archive/html/guile-devel/2016-07/msg00012.html
This reverts commit 2d4da30fde. This Gnulib update was causing failures related to timezones in stime.c. I tried to fix it by adopting the time_rz module from gnulib but that then caused other failures. We can try again later.
2d4da30fde