[tex-live] Platform & library check on macOS during TL 2018 installation

[Richard Koch]

In the early days of MacTeX, we installed some libraries in /usr/local/lib, primarily to support ImageMagick. Later we made it a principle never to install libraries, which can confuse users later on.

So I very much support Mojca's proposal to print an error message and fail is libc++ is absent on darwin 10, particularly if the message briefly explains how a user can solve the problem.

Dick Koch