Include files outside srcdir?

[Joseph Garvin]

I have a number of autotooled libraries. Currently their Makefile.am's
are largely redundant, so I thought I would use the 'include' keyword
and have them pull their common parts from some file. But as far as I
can tell, the include isn't being observed by automake -- instead it's
ignored and passed on to make, where it defines targets that make
doesn't care about (install-exec-hook, etc.). The documentation says
automake only pays attention to includes that start with $(srcdir) or
$(top_srcdir), but that's not possible in this instance, because each
library is its own project (recursive make isn't being used). Is there
anyway to do this? The only thing I can think of is having the include
define the regular make targets and bypass the help automake would
usually provide.