Created attachment 500 [details]
patch for problem
eigen3 has a pkg-config file for installing. Additionally generate one for usage when not installed but inside the build-tree. pkg-config can honour these files.
See attached patch.
I'm no expert about these configuration subtleties, so I'm willing to trust you on that one.
A very nice addition would be to write (or at least start it) a short documentation page listing all the facilities offered to the user to properly include Eigen in its project (with version checking).
The uninstalled.pc is used in Qt and embedded context since around 2012.