Add example for using the SigC++ library

[ruudbeukema]

I managed to get the SigC++ library to work with CPM.

Although libsigcplusplus project has a CMakeLists.txt file, the includes and library target name did not end up in a CMake variable right away. That required some fiddling.

I hope you like my contribution and I'm happy to hear if things can be improved.

[TheLartians]

Hey, thanks for the great example for adding SigC++ with CPM! While it looks like a very useful library at this point I think we should limit what libraries we add to the in-repo official examples to the most popular ones, to keep it tidy and reduce CI time. How about adding the example it to our snippets wiki instead?

[ruudbeukema]

Hi Lars,

Thanks for your reaction. I've added my snippet to the wiki.

Keep up the good work on CPM!

Cheers, Ruud

[ruudbeukema]

I'll close this pull request.