www/node6: set environment variables for host build

The Node.js build system supports cross-compilation via node-gyp, which
picks up CC, CXX, LINK, C*FLAGS, and LDFLAGS from the environment for the
target build, but not for the host. We need to set the environment variables
for the "host" build.

Sponsored by: Miles AS
Differential Revision: https://reviews.freebsd.org/D19250