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Backport (part of) r521207 | gerald | 2019-12-28 from lang/gcc9-devel:

Description

Backport (part of) r521207 | gerald | 2019-12-28 from lang/gcc9-devel:

Enable GCC plugins support by default.

PR: 242644
Submitted by: tobik
Differential Revision: https://reviews.freebsd.org/D22292