Include libmagic code?

[rstub]

Currently the CI builds on Mac OS are failing, most likely because of different libmagic versions. Possible solution:

• Include the libmagic source in the package since it is reasonably small and needed for Windows support anyway
• During configure configure and build libmagic if no usable installation is found
• On Windows always build the included sources, which have to be pre-configured, since Rtools are not sufficient to run configure

Open questions:

• What should be the default if a usable libmagic installation is found?