doc: gcc version is at least 4.8.5 in BUILDING.md

>= 4.8.5 is required because of compiler bugs in earlier versions

PR-URL: https://github.com/nodejs/node/pull/11840


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>