[Enhancement] Add cmake support

[terabyte25]

Self explanatory. Just an enhancement request to add support for cmake.

[VolkMilit]

@terabyte25, I appreciate you like my work, but I'm almost abandoned development of this project. Reason: I don't really have motivation to move on. And another (tes3mp-related) project eats all of my free time. Also, CuteMail isn't ready for everyday usage -- eml parser is broken, and code is outdated. I used old Qt less then 5.7, so some things might not work. Maybe someday I will return to development of this project.