protozero: Update to version 1.7.1

[frankdean]

Description

Re: https://trac.macports.org/ticket/64629

This is a header-only library.

One of the unit tests uses the Protobuf
library if it is found. I've added a protobuf variant to allow that test to be run. Other than that,
the Protobuf library is not required.

I have disabled treating warnings as errors to handle the reported issue.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 11.6.3 20G415 x86_64
Xcode 12.5 12E262

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?
• 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?