Yet another attempt at cross-directory dependencies. Seems to fix the parallel

Yet another attempt at cross-directory dependencies.  Seems to fix the parallel
build, and hasn't broken the out-of-date case so far as I can tell, so far...
Added a bunch of comments describing the cases that need to be handled.

* Makefile.in ($(BUILDTOP)/include/gssapi/gssapi.h, generic/gssapi.h,
generic/gssapi_err_generic.h, krb5/gssapi_err_krb5.h): Comment out old rules
and dependencies; depend on all-recurse and supply a no-op rule.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@15270 dc483132-0cff-0310-8789-dd5450dbe970