Write DEAL_II_SOURCE_DIR to Config.cmake

[peterrum]

While trying out #13033 (comment), I couldn't access DEAL_II_SOURCE_DIR. Is there any way to do that; if not can we export this variable?

[jppelteret]

I don't think that this is necessarily a good idea. Most package managers download the source to some temporary directory, so this could point to nothing in particular if the package manager cleans up after itself.

[peterrum]

I don't think that this is necessarily a good idea. Most package managers download the source to some temporary directory, so this could point to nothing in particular if the package manager cleans up after itself.

I didn't think about this. I guess the solution with DEAL_II_SOURCE_DIR regarding tests.h is not a general solution and the only options are to install the file or to continue to copy the file into the external projects. Sigh...