specifying include directory

I've got a package which contains many libraries, each with headers
that should be installed a subdirectory of includedir.  For example
in the Makefile.am for library "foo", I have:

    include_HEADERS = foo.h ...

In the Makefile.am for library "bar", I have: 

    include_HEADERS = bar.h ...

I would like to install the headers in $(includedir)/foo and
$(includedir)/bar.

I think that explicitly setting installdir to @includedir@/foo, etc.
in the Makefile.am's will work --- but before I do that I thought I'd
ask whether there a recommended best practice for this?

    --jtc

-- 
J.T. Conklin