[PROJ] Bump CMake version for PROJ 7.0

[Mike Taves]

Any further discussion? Is a bump to CMake 3.9 OK for everyone?

Note there was recently a bit of discussion on a GitHub ticket:
https://github.com/OSGeo/PROJ/issues/1858#issuecomment-576718862

On Tue, 14 Jan 2020 at 09:49, Mike Taves <mwtoews at gmail.com> wrote:
>
> Hi all,
>
> As was suggested in https://github.com/OSGeo/PROJ/pull/1542 this is a
> discussion to bump the minimum CMake version for PROJ 7.0
>
> 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.
>
> CMake 3.9.0 was released [2] July 2017, so is at least 2 years old for
> PROJ RFC 3's requirement.
>
> One feature from this version that I plan to implement is
> CheckIPOSupported [3] module. GoogleTest [4] was added for this
> release, however I don't think it would provide any advantage.
>
> Any other thoughts or suggestions on other CMake versions?
>
> Cheers,
> Mike
>
> [1] https://cliutils.gitlab.io/modern-cmake/chapters/intro/installing.html#cmake-default-versions
> [2] https://blog.kitware.com/cmake-3-9-0-available-for-download/
> [3] https://cmake.org/cmake/help/v3.9/module/CheckIPOSupported.html
> [4] https://cmake.org/cmake/help/v3.9/module/GoogleTest.html