Work around some ULTRIX make bugs.

Work around some ULTRIX make bugs.

ULTRIX 4.5 make fails during rule lookup when it tries to build a
target in a nonexistent directory that might have inference rules
associated with it (i.e., ends with a defined suffix), for example:

  % echo 'nonexistent/baz.o: ;' | make -f -
  Directory nonexistent/: Make: Cannot open.  Stop.

We can work around this problem by using a prerequisite to create the
output directory instead of trying to create it directly in the rule.
Automake actually does this already for its own rules, so let's try
to hook the same mechanism.

Pull in similar fixes from dxcommon.