[PATCH] Make -fdiagnostics-color= default configurable, default to =auto

[Manuel López-Ibáñez]

I think it is great that =auto becomes the default. However, adding a
configure option for this seems overkill. If anyone really really
wishes to change the default, they can simply edit a single-line in
common.opt.

Apart from that, I really cannot understand why someone would want the
options none, auto or always: none is equivalent to auto-if-env but
without the "if-env" escape route, and neither auto nor always allow
disabling the coloring globally (which is precisely what users hating
coloring are going to be asking for and they won't be happy if we tell
them to recompile gcc).

I think there are only two defaults that matter:
auto-if-env-otherwise-never (the current one) and
never-if-env-otherwise-auto. I wish we had chosen the second one
earlier, but this is not such a critical feature to not change the
default now. With never-if-even-otherwise-auto, an empty GCC_COLORS
(or a value ="never") could disable coloring. Currently,
GCC_COLORS="never" actually enables coloring (well, any value does).

Cheers,

Manuel.