Fix shocco's requirement for a markdown command

[telemachus]

schocco requires a markdown command. The previous version of the formula
checked for the markdown formula. However, there's an edge case involved.
The discount formula installs a command markdown which can also satisfy
shocco's requirement. (And I would add that discount's implementation of
the markdown command is far superior to markdown's, though I grant that's
debatable.)

This update checks for any markdown-providing install, and if one is not
found tells the user what options are available.

I think this is better than just installing the markdown formula. First, if
discount is already installed, then when brew tries to install markdown,
the linking stage will fail. Although the shocco install as a whole will go
through, the failure message about linking markdown is likely to confuse
the user. Also, if discount is already installed, that problably means the
user doesn't want the markdown formula. Finally, this way of handling
things is more easily extended if other formulas become available which
provide a valid markdown command.

[mistydemeo]

Pulled minus the comment, thanks a lot!

[cdlm]

It does not seem to work:

$ which markdown
/opt/homebrew/bin/markdown

$ brew install shocco
shocco requires a `markdown` command.
…

[telemachus]

shocco was just moved to the "new requirement syntax". I'm guessing it has something to do with that, but I have no time to check right now. See these two commits though:

• 3ad7174
• 43f2bfa

Edit: Yeah, if you roll back the change in 3ad7174, then it works. I don't know how the new syntax works, however, so I'm not sure where the real problem is.

[jacknagel]

f863ea9

Requirements are now evaluated in an environment identical to that of the build (since that is what matters), so most things that just check for something in $PATH need to opt-in to env :userpaths.

[cdlm]

@jacknagel still does not work, shocco's makefile tries to install its own copy of markdown even though I have discount installed and $(brew --prefix)/bin/markdown exists… I suppose configure is not getting the correct environment. I also have ronn installed using rbenv and that's not detected as well (but then it's ~/.rbenv/shims/ronn so that's a little far-fetched)

See #18516