py-frozenlist: fix build with new pythons on old systems

[barracuda156]

Description

Fix

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.6
Xcode 3.2

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@tobypeterson for port py-frozenlist.

[tobypeterson]

I don't really love this approach, I'm curious how this impacts building on current versions of macOS/Xcode, which default to C17.

[barracuda156]

@tobypeterson I works fine, which is seen both from CI here and from multiple non-Python ports which use this flag.

However, fixing this via Python PG is a better way, it is impractical to add identical per-port fixes. So I close this in favor of a PR for PG: #26830

[tobypeterson]

Right, I'm not suggesting that it won't build - but building with an older C standard can slightly affect things, though usually in relatively meaningless ways.