Appending to builtin Automake variables from an included file

[Allan Caffee]

What is the cleanest to way to append something to a builtin variable
(e.g.  MAINTAINERCLEANFILES) from an Automake "header".  For example I
want all of my Makefiles to delete the Makefile.in by the
maintainer-clean target.  The way I currently do this is:

        --- am/global.am ---
GLOBAL_MAINTAINERCLEANFILES = Makefile.in

         --- Makefile.am ---
include $(top_srcdir)/am/global.am
MAINTAINERCLEANFILES = $(GLOBAL_MAINTAINERCLEANFILES)

Which of course creates more work than it saves when it comes to
deleting one file.  But this has many potential uses when you consider
including an Automake header that builds an RPM or doxygen
documentation.  Is there a cleaner, ideally non-invasive method for
adding things to these builtin lists?

~Allan