games/netradiant: fix build with GCC-based architectures

Add USES=compiler:c++11-lang to fix linking with boost.

Also replace ARCH on powerpc64 with powerpc to fix packaging.

While here, add USES=gl gnome localbase:ldflags.

PR: 237962
Approved by: danfe (maintainer timeout), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D20612