License Exceptions

Description:

There is software out there with license exceptions, and SPDX allows for this with the WITH syntax, e.g. GPL-2.0-or-later WITH GCC-exception-2.0

Additional info:

• SPDX license list (introduction to license exceptions can be found here).
• SPDX exceptions list (list of all known SPDX exceptions).

Steps to reproduce:

1. Add an exception to licenses= in PKGBUILD.
2. pkgctl build
3. Watch namcap spit out issues for licenses.