DISPATCH-1268 - Set cmake_minimum_required to VERSION 2.8.12

[jirkadanek]

A lot of changes from PR #288 is compatible with CMake 2.8.12 and can therefore still be used by the project.

This PR contains only the first change, to cmake_minimum_required version, to see if there is a chance to get it in.

[codecov-io]

Codecov Report

Merging #449 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #449      +/-   ##
=========================================
- Coverage   86.84%   86.8%   -0.05%     
=========================================
  Files          85      85              
  Lines       18746   18746              
=========================================
- Hits        16280   16272       -8     
- Misses       2466    2474       +8

Impacted Files	Coverage Δ
src/container.c	77.4% <0%> (-0.76%)	⬇️
src/router_core/agent_link.c	63.84% <0%> (-0.57%)	⬇️
src/router_core/core_client_api.c	91.57% <0%> (-0.37%)	⬇️
src/parse.c	88.29% <0%> (-0.26%)	⬇️
src/router_core/route_tables.c	76.67% <0%> (-0.25%)	⬇️

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[ted-ross]

I think this update is OK. The oldest downstream environment that I'm aware of is RHEL6, which appears to use cmake 2.8.12.

[jirkadanek]

All right, I'll try to find more useful stuff to improve, given 2.8.12 features can be used.

[kgiusti]

I'm good with this.
Ubuntu Trusty has 2.8.12 and that's the oldest LTS that's still officially maintained.

[jirkadanek]

I think this is as far as I am going to go at this point. There are still some tweaks in the original PR which I did not do here, so I might open new PR for that, but this PR seems to me has enough changes already.

I can squash the commits if you'd like.

[jirkadanek]

I'll merge this later today if no one objects. Thanks.