Fixed "use" dependencies in cmake target. No version checking.
Merge pull request #63 from jemc/master

Problem: According to gsl docs, 'include' is deprecated.
Merge pull request #65 from mditzel/master

Issue #64: Fixed "use" dependencies in cmake target.
Update .pc file generation with version information.
Generalize use project dependency detection, apply ordered detection, remove package_dependency as redundant.
Merge pull request #66 from pmienk/master

Removal of `package_dependency`, rationalization with `use`
Merge with origin/master