cmake support / interface library commits are broken

[tamaskenez]

The commit 7b10a08 (resolves #222) adds a CMake INTERFACE library which breaks the build for older CMake versions.
The commit 1ef6415 tries to fix this but it's still wrong. The INTERFACE library has been introduced in CMake 3.0.

I'd like to draw your attention to PR #239 which solves this and other, related issues and still runs correctly with older CMake versions. I suggest you to revert 1ef6415 and 7b10a08 and merge #239 instead.
#239 has been in the queue for almost 3 months. I guess you had some problems with it, that's why you merged 7b10a08 instead. Please let me know how to improve #239 if that's the case.

[AzothAmmo]

I don't think there is any problem with #239, I just missed it when I was organizing things recently - I'll revert and take it instead.

[AzothAmmo]

merged #239