ports.7: Add an example of how to use flavors
At the moment the manual page is not documenting how to build
a flavored package. Let's start documenting flavors with
an example of a typical use case.
Reported by: cem, dim
Reviewed by: bcr, cem, mat, matthew
Approved by: cem (src)
Differential Revision: https://reviews.freebsd.org/D19531