[autotools,release] Install autoconf macros to $(datadir)/dune/aclocal

Previously, the macros were installed to $(datadir)/aclocal. As we have
some macros in DUNE that are also installed for some Linux distributions
this caused conflicts in package managers. With this patch we install
them to $(datadir)/dune/aclocal to resolve this issue outlined in flyspray
task 1409
https://dune-project.org/flyspray/index.php?do=details&task_id=1409
(cherry picked from commit fe4ffa0e34cbc5f44837d3646c7c340d3b337218)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>