Bump required automake version to 1.11

see https://www.gnu.org/software/automake/manual/html_node/Obsolete-Macros.html
and http://lists.gnucash.org/logs/2013/10/2013-10-27.html#T16:03:07

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23341 57a11ea4-9604-0410-9ed3-97b8803252fd