www/node: Update to 6.9.1

Bump to the latest upstream 6.x release. Node.js v6 is now an LTS
release, see the v6.9.0 release announcement for details. The v6.9.0
release was also a security release, while v6.9.1 fixes a regression
introduced in v6.8.0.

Cleanup clang vs. gcc handling by using USES=compiler:c++-lib

https://nodejs.org/en/blog/release/v6.9.0/
https://nodejs.org/en/blog/release/v6.9.1/
https://nodejs.org/en/blog/vulnerability/october-2016-security-releases/

PR: 213604
Security: CVE-2016-5180