Redirecting the stderr of Dune causes recompilation of C stubs

[nojb]

Dune decides whether to pass -fdiagnostics-color to the (GCC-style) C compiler or not depending on whether stderr is a terminal or not. This means that if one does dune build 2>/dev/null followed by dune build, the second command will rebuild all C stubs, which seems wrong.

One way to approach this may be to always make pass the -fdiagnostics-color flag to GCC, but filter out the ANSI codes in Dune, like it does in other cases (if I understand correctly).

[nojb]

cc @MisterDA

[anmonteiro]

This is only tangentially related, but it looks like we have the same issue for the OCaml compilers:

dune/src/dune_rules/global.ml

Lines 8 to 11 in d9b0e2e

	(let env =
	if (not capture_outputs) || not (Lazy.force Ansi_color.stderr_supports_color)
	then Env.initial
	else Colors.setup_env_for_colors Env.initial

e.g. in OCaml 5.2 where Misc.Style.inline_code is either bold text or quoted text depending on !Color.enabled, you can write a cram test that behaves differently according to whether it's piped to cat or not