CMake: Force GCC/Clang color diagnostics for Ninja (#1596)

[maxrdz]

Issue description

Linux users are recommended to use Ninja when generating the build files through CMake. Ninja buffers output, so tools are writing to a pipe, not directly to the terminal, so auto-detection may turn ANSI colored output off conservatively.

Solution description

This PR forces color diagnostics by setting CMAKE_COLOR_DIAGNOSTICS to ON on CMake versions 3.24 and above. For older CMake versions, compiler flags are invoked manually for GCC and Clang.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[maxrdz]

This resolves #1596.

[Moguri]

Maybe instead of a new flag and trying to support older versions of Cmake, should we just change the default for CMAKE_COLOR_DIAGNOSTICS for relevant generators and CMake versions?

[maxrdz]

Maybe instead of a new flag and trying to support older versions of Cmake, should we just change the default for CMAKE_COLOR_DIAGNOSTICS for relevant generators and CMake versions?

The CMAKE_COLOR_DIAGNOSTICS option is relatively new to CMake (version 3.24.x) so I think it would be a safe route to take if we first verify the version of CMake being used.

[maxrdz]

I do agree that we can just always apply it for Ninja gen builds and remove the new CMake option, since it should not affect anything with the build unless someone doesn't want the compiler to print ANSI colors.

[rdb]

I'm OK with this, but can we make it a cache variable, so that users can disable it? And if people turn it off, we shouldn't add the compiler option.

[maxrdz]

Sorry for the late update, was busy for a few weeks. :)

[Moguri]

I am okay with this as well.