Work around stupid automake dependency-tracking bug

Work around stupid automake dependency-tracking bug

bison/flex-generated objects, when being run in a VPATH build with
--disable-dependency-tracking (i.e. Gentoo), would fail to be created
because automake didn't bother creating the destination directories
before trying to create the objects.

Fix this by depending on the destination directory stamp, which
according to the automake mailing list, should hopefully remain fairly
stable.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>