Much better support for ninja cmake backend

[jgfouca]

Description

Add much better support for different cmake backends, specifically ninja.

Change list:

1. Instead of hardcoding gmake invocations, use cmake --build . which will flexibly work for any backend
2. Add arguments for picking a specific backend to case.build and create_test
3. env_mach will automatically add ninja to PATH if it's there.
4. Instead of using CIME_SHAREDLIB_NINJA as a magic var, use the env that is known to CMake (CMAKE_GENERATOR).
5. Add machine setting / case env var CMAKE_BACKEND

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation

[github-actions]

You can preview documentation at https://esmci.github.io/cime/branch/jgfouca/more_ninja_stuff/html/index.html

[Copilot]

Pull request overview

This PR improves CMake build backend flexibility (with a focus on Ninja) by shifting away from hard-coded GNU Make invocations and adding configuration / CLI support for selecting a backend, along with documentation and schema updates to support a new CMAKE_BACKEND machine setting.

Changes:

• Replace direct gmake/ninja calls with cmake --build in several build paths to better support multiple CMake generators.
• Add CLI options to select Ninja vs gmake in case.build and create_test, plus a new machine/case setting CMAKE_BACKEND.
• Update machine env handling to expose Ninja on PATH when vendored, and update schemas/docs to include CMAKE_BACKEND.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
doc/source/ccs/model-configuration/variables/machine.rst	Documents new CMAKE_BACKEND machine variable in the machine variables reference and schema examples.
doc/source/ccs/building-a-case.rst	Documents CMAKE_BACKEND as a build-affecting variable and adds a note about backend availability.
CIME/XML/env_mach_specific.py	Prepends vendored Ninja to PATH during env setup for cases.
CIME/Tools/case.build	Adds --ninja/--gmake flags and uses CMAKE_BACKEND defaulting behavior when no flag is provided.
CIME/test_scheduler.py	Propagates backend selection into scheduled build phases by appending --ninja/--gmake to case.build invocations.
CIME/SystemTests/hommebaseclass.py	Switches HOMME build/run targets to use cmake --build rather than direct make invocations.
CIME/scripts/create_test.py	Adds backend selection flags and passes them through to the test scheduler.
CIME/data/config/xml_schemas/config_machines.xsd	Adds CMAKE_BACKEND to v2 schema.
CIME/data/config/xml_schemas/config_machines_version3.xsd	Adds CMAKE_BACKEND to v3 schema.
CIME/data/config/xml_schemas/config_machines_template.xml	Documents optional CMAKE_BACKEND in the machine template.
CIME/build.py	Refactors model and clean build commands to use cmake --build and uses CMAKE_GENERATOR instead of a CIME-specific Ninja env var.
CIME/build_scripts/buildlib.cprnc	Uses cmake --build for cprnc build step instead of gmake.

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[bartgol]

Looks good to me. I would also consider renaming GMAKE_J to something like BUILD_J...

[bartgol]

Should we check that only one between gmake and ninja is true?

[jgfouca]

Yes, that happens at the case.build level. I can add it here too.

[fischer-ncar]

Looks good.