Teach mono to accept `USES= mono:build` as optional runtime dependency.

[dch]

Summary

• updated PHB in https://reviews.freebsd.org/D23398
• updated reviewers after taking ownership of databases/foundationdb port

Test Plan

• confirm via poudriere that database/foundationdb does the

right thing.

root@pkg:~ # pkg info
boost-libs-1.72.0              Free portable C++ libraries (without Boost.Python)
foundationdb-6.1.8_4           Distributed, transactional key-value store
icu-65.1,1                     International Components for Unicode (from IBM)
libeio-4.18_2                  Full-featured asynchronous I/O library for C
pkg-1.12.0                     Package manager

• find a small mono port & confirm it still has the runtime

dependency - devel/log4net

 pkg install -r skunkwerks devel/log4net
Updating skunkwerks repository catalogue...
skunkwerks repository is up to date.
All repositories are up to date.
The following 12 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
        log4net: 1.2.10_3 [skunkwerks]
        mono: 5.10.1.57_2 [skunkwerks]

[prj_rootwyrm.com]

This makes perfect sense to me. Some ports require Mono as a runtime (because they get called as 'mono blah.exe') but some ports don't, and Mono is a pretty hefty build.