Using Variables in *_SOURCES

[Seth Greenfield]

Hello,
      I understand that you cannot place any type of
variable in a _SOURCES variable. My problem is that I have
multiple applications which compile code from a common code
base. However, I cannot create a library of this common
code, because the code makes references to the individual
application. 

I would like to be able to have a single file which the
makefiles for each application reference containing the list
of files that it needs from the common code base.

For Example, if the common code base contains A.cpp, B.cpp,
and C.cpp, which and I wished to add D.cpp, I would like to
be able to add this in only a single listing, as opposed to
the makefiles in each Application. I would like this listing
to be in a single file, independent of each application.

Just a note, each application compiles all the code in the
common code base.

Thank you,
Seth