contrib: libarchive: disable auto dependencies on Darwin

When building on Darwin, CMake checks for dependencies in non-standard locations like the Homebrew and MacPorts installations without any easy way to opt out of this, apparently. In addition, it does not properly set the linker flags to these paths if it end up finding a library in there so it just results in a linker error in the end.

To work around this, disable all autodetected dependencies that we do not provide as contribs.