Minimum CMake version for PROJ 9.9 series

[mwtoews]

Every couple of years, it's generally good to increment CMake's minimum version used to build PROJ. It was 2 years ago for PROJ 9.4 that the minimum CMake was changed from 3.9 to 3.16 (#3997).

This issue is a general discussion to see if we can agree if PROJ's minimum CMake version should be increased for the PROJ 9.9.x series (anticipated September 2026), and what version it should go to. Note that RFC 3 says that "software packages will always be at least two years old", so the upper limit could be CMake 3.29-ish (released March 21, 2024).

Refer to What’s new in CMake for a handy/quick view of features from CMake releases.

For example, I see a decent case in #4753 that the minimum should be moved to 3.18 (or even 3.22). Are there other case examples that would benefit from other modern versions of CMake?

[dbs4261]

There's a lot that would be helpful coming up in CMake, eg. 3.31-4.3+ that PROJ would benefit from, both in terms of packaging and codegen. But that will need to live in another branch because 3.29.2 is, as you said, the 2 year limit.

Keeping that limit in mind, and starting at 3.29 these are the useful features:
3.27: Big generator expression improvement, added $LIST:... and $PATH:... as well as $<INSTALL_PREFIX> allowed in install(CODE) and install(SCRIPT). Theres also some important improvements to getting a targets runtime dependencies on Windows which simplifies packaging those DLLs with PROJ on Windows platforms.
3.25: The BLOCK command was added to manage variable and policy scopes.
3.24: FetchContent integrates with find_package which makes optional vendoring a lot cleaner. Also adds a bunch of generator expressions for preparing MacOSx Frameworks.
3.23: Introduces file sets, a huge update to how headers are handled.
3.21: Is required to request C23, used for #embed. Big updates to the install command for handling runtime dependencies. Custom targets also get a better understanding of dependencies.

Now, I'd love to just jump to 3.29 at that 2 year limit, but I think that might provide a bit too much friction for some people. In system package managers Ubuntu 22.04 provides 3.22.1 and 24.04 provides 3.28.3. Debian, on the other hand, is much better about providing updated versions in their backports repository, and Arch seems to stay up to date.

Based on all this, I'd say jump to 3.27.