ENH: Add BUILD_SHARED_LIBS to macOS build CI configuration

[thewtex]

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[mathstuf]

Do: check

[mathstuf]

Ah, this is ignored because it targets the python-builds branch. @bradking

[bradking]

@thewtex didn't you ask for python-builds to be a nightly follower of master? There should be no PRs targeting it directly.

[thewtex]

python-builds is currently used to trigger Python builds. But, this configuration is a WIP and may changed.

[hjmjohnson]

Please rebase after the VXL updates in #175 are in place.

[thewtex]

We prefer static builds on macOS.