Remove --upgrade-cabal recommendation

[snoyberg]

As we started discussing on Slack, and related to #2795. The original issue is no longer present in Stack due to the usage of custom setup deps now being fully supported. If I manually remove the custom-setup stanza from the glib cabal file, the problem reappears. In theory, the right solution is to notice that there's a Cabal library present in the snapshot database and use that instead. I don't want to do that though, because:

1. It will add extra code (and possibly a performance hit) to do all of the checking for Cabal library versions. I think this information is already loaded up by Stack, but if not, it's an extra ghc-pkg call, which isn't cheap.
2. It adds some level of non-reproducibility to build plans, as the results will depend on whether we first ran stack build Cabal or not.

I've instead added a warning in the case of a missing custom-setup stanza. I believe this is the right approach: all of these build issues will disappear (in theory) with properly provided custom-setup, so we should be encouraging them.

I'd go so far as to say that, in the not-too-distant future, removing the ability to upgrade the global Cabal library entirely may be the right approach.

Pinging @lwm @borsboom

[decentral1se]

all of these build issues will disappear (in theory) with properly provided custom-setup, so we should be encouraging them.

Ah right. I wasn't aware.

Thanks for the explanation.

I'd go so far as to say that, in the not-too-distant future, removing the ability to upgrade the global Cabal library entirely may be the right approach.

Have you seen #2995? Please do weigh in there if you think it's not something useful.

[borsboom]

Sounds good