[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

m4/Makefile.am

@@ -6,7 +6,7 @@ ALLM4S = 					\
 	superlu-dist.m4				\
 	superlu.m4
 
-aclocaldir = $(datadir)/aclocal
+aclocaldir = $(datadir)/dune/aclocal
 aclocal_DATA = $(ALLM4S)
 
 EXTRA_DIST = CMakeLists.txt $(ALLM4S)