Add macosx11.1 support to configure

[philburk]

This is needed to support the new XCode 12.3.
I also went ahead and added macosx11.2 because it is surely coming.

Also changed macosx-version-min to 10.6

Fixes #378
Fixes #468

[Be-ing]

Why is there an upper limit on the supported SDK version?

[philburk]

Why is there an upper limit on the supported SDK version?

@Be-ing - What limit are you referring to? Are you asking why I stopped at "11.2"?

My answer would be that the elif chain is cumbersome. In the past we only supported released SDKs. I added 11.2 in anticipation of the next release. How many is enough? I don't know.

I also added a TODO that suggested using -showsdks to avoid the chain of elifs and also work automatically with some future releases. But that requires a bit more study. This was a quick fix for a broken build.

[Be-ing]

You can use xcodebuild -version -sdk macosx SDKVersion to get the installed version. Just check that is >= 10.6 and this PortAudio release will remain buildable on future versions of macOS.

[philburk]

Thanks! I need an emoji for head exploding.
I will experiment with that for an alternative PR.
I may go ahead and just merge this as an emergency fix.

[Be-ing]

https://emojipedia.org/exploding-head/

[philburk]

I am closing this in favor of #470. It is much simpler and will last longer without needing changes.
By the time it gets to the new code it should be SDK 1.6 or later.