cmake AUTHOR_WARNING if linux distribution doesn't match

[smanders]

there have been issues when a developer on a given linux distribution uses an externpro built on a different distribution (or a different version - for example Ubuntu 16.04 vs 18.04)

record lsb_release -d in the XP_INFOFILE (.txt file at the root of the externpro installation) and then add some cmake to the find script to output a message(AUTHOR_WARNING if the distributions don't match

[smanders]

completed with commits to dev branch referenced above

[smanders]

just a note about a thought that I had related to this issue... (future enhancement)

• if we wanted to record something similar to lsb_release -d which is linux-specific, on Windows we could use systeminfo and record the OS Name: and/or OS Version: lines
• systeminfo gives results back in the terminal, which we could parse out... winver pops up a GUI

[smanders]

this warning will serve as a reminder that building externpro and using externpro on two different distributions is unsupported - and the issue in #210 would have been easier to spot