graphics/opencolorio: fix build with GCC-based architectures

Add -Wno-unused-function -Wno-error to CXXFLAGS when using GCC.

While here, add USES=localbase:ldflags.

PR: 233888
Approved by: FreeBSD@ShaneWare.Biz (maintainer), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20089