Back in commit cf9f3452 (cmake: bump minimum version to 2.8.11,
2017-09-06), we have bumped the minimum CMake version to require at
least v2.8.11. The main hold-backs back then were distributions like
RHEL/CentOS as well as Ubuntu Trusty, which caused us to not target a
more modern version. Nowadays, Ubuntu Trusty has been EOL'd and CentOS 6
has CMake v3.6.1 available via the EPEL6 repository, and thus it seems
fair to upgrade to a more recent version.

Going through repology [1], one can see that all supported mainstream
distributions do in fact have CMake 3 available. Going through the list,
the minimum version that is supported by all mainstream distros is in
fact v3.5.1:

	- CentOS 6 via EPEL6: 3.6.1
	- Debian Oldstable: 3.7.2
	- Fedora 26: 3.8.2
	- OpenMandriva 3.x: 3.5.1
	- Slackware 14.2: 3.5.2
	- Ubuntu 16.04: 3.5.1

Consequentally, let's upgrade CMake to the minimum version of 3.5.1 and
remove all the version CMake checks that aren't required anymore.

[1]: https://repology.org/project/cmake/versions