[PROJ] Bump CMake version for PROJ 7.0

[Mike Taves]

On Sat, 1 Feb 2020 at 04:25, Even Rouault <even.rouault at spatialys.com> wrote:
> Mike mentioned in his inital message
> """
> Based on the default versions of CMake [1], I can see that CMake 3.9
> is installed on most development systems, including Travis CI, so this
> would be an ideal minimum.
> """
>
> but looking now at [1]
> https://cliutils.gitlab.io/modern-cmake/chapters/intro/installing.html#cmake-default-versions
>
> it seems to have been updated very recently to mention 3.12.4 and looking at
> actual Travis logs at
> https://api.travis-ci.com/v3/job/281868129/log.txt
>
> I can see that 3.12.4 is indeed installed

Yes, this was updated a few days ago:
https://gitlab.com/CLIUtils/modern-cmake/commit/ad364ca2d92cb06747755aeb4c735648bd0e3931

That's great for our Travis CI tests, but not as a suitable minimum
version for RFC 3, as it's less than 2 years old. As I had mentioned
previously, the newest 2-year old version is CMake 3.10, however I
don't see any compelling features from this release to recommend it as
the next minimum version.