This allows for port testing with lang/go-devel via GO_PORT, setting
up the Go build environment in a single place, and is step one in
simplifying Go ports that often define too complicated do-build
targets themselves.
USES=go gains new arguments 'run' to add lang/go to RUN_DEPENDS and
'no_targets' for ports with composite builds that call 'go' themselves
and do not need the do-build/do-install targets of USES=go.
PR: 238849
Submitted by: dg@syrec.org (also D20745)
Reviewed by: mat, tobik
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D20746
