Update CMAKE minimum version to 2.8.12

[smburdick]

To remove the CMAKE build deprecation warning, update the minimum required version.

[miek]

Could this be updated to 2.8.12 as the warning suggests, or is there a reason why it should be updated to a higher version?

[smburdick]

Could this be updated to 2.8.12 as the warning suggests, or is there a reason why it should be updated to a higher version?

It could. I had no reason to make it that specific version. I can retry with 2.8.12.

[martinling]

The minimum supported version has been bumped again, so we now get:

 CMake Deprecation Warning at libhackrf/CMakeLists.txt:24 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of CMake.

I guess we can probably bump to 3.5 at this point (released 2016).

[mossmann]

I think 2.8.12 is good enough for today's release. Thank you for the contribution!