How to define a shared submodule?

[SoftCalc]

I have a bit of a dilemma. I'm working on two completely separate
projects for two different platforms (one is cross-compiled). They are
also relatively large projects with many sub-directories. The problem
is, they need to share some common code. The common code is self
contained in it's own directory.

Right now I've manually copied the sub-directory of the common code
from one project to the other on my local machine. This is a hack
solution filled with pitfalls. For one, updating the project that has
the copied sub-directory doesn't update that sub-directory. I have to
remember to manually update the sub-directory as well. This is error
prone. If I forget to manually update the sub-dir. my code will be out
of sync. Another problem, someone can't simply check-out the project
that has the copied sub-directory and build it. They need to know that
there is a directory from another project they need to copy over.

Has anyone come accross this problem before, and if so did you find a
reasonable solution? Is there any way to define an alias that contains
a module, and have that module buried several levels deep in the
project directory?