cmake: enable coloured compiler output (IDFGH-7449)

[MacDue]

This adds -fdiagnostics-color=always to the default compile_options,
without this GCC won't enabled colored output when called from
ninja (via idf.py), making errors much harder to decipher.

[MacDue]

This is something that has bugged me quite a few times when building with idf.py

[igrr]

Hi @MacDue, thank you for the PR.
Unfortunately this approach will enable color output also when the build system output is not a terminal which understands ANSI escape sequences.

To solve this issue, we plan to enable color diagnostic output in the following manner (ref. #4162 (comment)):

• upgrade CMake to the version which supports CMAKE_COLOR_DIAGNOSTICS environment variable — done in 2ee43cf
• in idf.py, check if stdout is a TTY and set CMAKE_COLOR_DIAGNOSTICS=1 if so
• IDEs which call CMake directly without idf.py (such as CLion or Eclipse) can set CMAKE_COLOR_DIAGNOSTICS if they support ANSI escape codes. For example, CLion has recently shipped this feature: https://blog.jetbrains.com/clion/2022/05/clion-starts-the-2022-2-eap/
• in headless builds (CI, or when output is redirected to another program or a file) color output will stay disabled.

If you have any suggestions, please leave a comment in the linked issue about color diagnostic output. Thanks again!

[MacDue]

While I appreciate a better fix is coming, I still think it's silly this has been disabled for so long. It's trivial to remove ANSI sequences from the output after the fact with something like ansi2txt, but impossible to add them & makes for unpleasant error messages.

[igrr]

Support for color diagnostics is now available in master, please see #4162 (comment). I'm sorry we did not merge your PR as is.