automake & missing subdirs

[Jason Kraftcheck]

I am working on the development of a set of libraries.  The core of the
code is LGPL, but it includes several optional modules that require
proprietary libraries.  The problem with the modules using proprietary
libraries is that the company that owns those libraries requies a
license agreement stating that we will not distribute any of our own
code that includes any of their headers, calls any of their functions,
etc. except to other licensees of their library.  So we'd like to
distribute our library source both with and without our source for
accessing the proprietary libraries.  So our source tree will have
optinion subdirectories.  Is there any way to define things such that
automake a) doesn't fail when it cannot find the Makefile.am and b)
ignores entiries in the SUBDIRS (in the Makefile.am in the parent
directory) for subdirs that do not exist?

thanks,

- jason