Switch to CMake.

[Web-eWorks]

This PR (partially) resolves (github u no close) #4461, by moving travis builds to CMake over autotools.

We're still using the thirdparty repo for travis - is that needed? It would shave a good 5-10 minutes off our already lengthy compile times if we were to drop it.

Additionally, this PR fixes #4466.

[Web-eWorks]

So I got most of this to work, but travis apparently has some issues that no-one else is experiencing. I estimate a few more commits before this is ready to merge.

[laarmen]

Just a heads up, there are conflicts plus a travis build fail.

I'm not against using CMake as a default build system, but I'd rather keep the autotools around for a bit longer, just to be sure we're not breaking anybody's setup (everybody doesn't follow the github project and simply pull regularly from master).

Thanks for the hard work on the issue!

[Web-eWorks]

@laarmen thanks for the report. I plan on keeping autotools in a deprecated state for a while before we remove it outright.

I think I may have a solution to the latest round of errors, so this might actually be ready for a merge soon.

...I think.

[Web-eWorks]

Huzzah, huzzah! It builds! Finally, everything works again.
@impaktor: pending a quick review (and maybe a squash), this PR is ready to merge. The deploy script needs updating to use the new binary location, but I'll take care of that when we add the API key.

[impaktor]

I'm without pioneer-computer for the rest of this week, so I can't test it, but if @fluffyfreak and @ecraven, who are the main guys working on the C++ source the past year(s), are happy, then so am I (assuming squash 😄).

[impaktor]

So we already have cmake in master, right? So this moves travis to also use cmake as well, and fixes the aforementioned bug? Is there anything for us to test? If not, we could merge now-ish, or by the weekend at the latest.

[Web-eWorks]

@impaktor This just makes Travis build with CMake instead of autotools, updates the CMakefile so that we can build with thirdparty, and restores compatibility with versions of CMake < 3.12. It is ready to merge now, if you wish.