[PROJ] Bump CMake version for PROJ 7.0

[Mike Taves]

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