doc: suggest xcode-select --install

If you don't have the full Xcode installed, the command-line tools are
all you need.

PR-URL: https://github.com/nodejs/node/pull/13264
Fixes: https://github.com/nodejs/node/issues/6449
Refs: https://github.com/nodejs/node/pull/9215


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>