common source code folder shared by all projects in a project file

There is already the notion of a "Library" or an "Object File" project in CrossStudio, but these don't seem to extend well to the described scenario. Both pre-compile their source code using the settings, user include path, etc. of that particular project (rather than the project that is dependent on them). If there are multiple configurations, each of those configurations must be manually maintained in lockstep with the superset of all projects that depend on those "Library" or "Object File" projects.

What is being suggested is some sort of common folder:

<solution>
<project>
<folder />
...
<folder />
</project>
...
<project>
<folder />
...
<folder />
</project>
<common />
</solution>

In the above pseudo-code, each project has one or more source code folders. The overall solution then has a "common" (for lack of a better name... it could just as easily use the same, existing "folder" tag) folder.

The key difference to "Library" or "Object File" is that each of the projects compiles code as if the "common" folder were one its own folders.