Add package config file generation for find_package

[a-pekar]

(without package config version file)

[a-pekar]

Please, review. Travis build seems to have failed for some irrelevant reason

[dascandy]

Travis build fails on Travis CI no longer being able to find the Clang-3.7 package for their majorly outdated Linux deployment - probably because it's gone out of LTS support a few years ago. I'll check if I can move it to more up to date Clangs and distros. No worries, none of your code was even looked at in the failed build, and all builds that were run are successful.