Support aclocal flags

We can't create Cutter supported configure.in when Cutter isn't
installed with --prefix=/usr.

With this change, we can use Cutter installed by --prefix=/tmp/local
by the following command line:

    % export ACLOCAL_FLAGS="-I /tmp/local/share/aclocal"
    % ./autogen.sh
    % ./configure --help | grep cutter
      --with-cutter           Use Cutter (default: auto)