Won't run or compile on macOS Catalina

[markaudacity]

What minimum version of macOS does FluxEngine need to run? I can't find that information anywhere here or on cowlark.

[davidgiven]

I'm actually not sure --- I'm not deeply into OSX. I only have two versions to test with, which is my up-to-date test machine and whatever version Github CI is using.

[markaudacity]

The specific error gmake throws is: lib/config.cc:610:13: error: no viable constructor or deduction guide for deduction of template arguments of 'shared_ptr' std::shared_ptr(FluxSource::create(_verificationFluxSourceProto));