POSIX issue 7 (2017) now required

[gperciva]

The recently-added CHECK_POSIX_SH [1] uses command -v. This is widely accepted as the proper answer to "how do I portably check if a command exists" [2], and has been part of our make test since 2016-04-23 [3].

However, -v only became mandatory in issue 7; previously, it was a [UP] User Portability Utilities option [4]. The debian posh shell does not support it, for example. Solaris is inconsistent between the shell built-in command, /usr/bin/command, the man-page, and /usr/xpg4/bin/command (the latter one is fully compatible with POSIX issue 7, as far as I can tell).

I'm still investigating, but thought that I should document the issue.

[1] abb7241
[2] https://stackoverflow.com/a/677212
[3] 4dc4eeb
[4] https://pubs.opengroup.org/onlinepubs/009695399/utilities/command.html

[gperciva]

Resolved in #242.