1. Raise an exception on macOS.
2. Verify that the missing libraries are specified either as Strings or
Regular Expressions.
Signed-off-by: Maxim Belkin <maxim.belkin@gmail.com>
We have strings containing hardcoded ".dylib" extensions in homebrew-core.
To be able to bring linuxbrew-core and homebrew-core closer together,
I am introducing a new generic attribute that can be used in formulae.