win: fix custom actions for WiX older than 3.9

This is a port of e192f615

 .

Original commit message:

  Older WiX versions included a header with extern "C" declaration,
  hence the custom action source must be C++.

Reviewed-By: João Reis <reis@janeasystems.com>
  PR-URL: https://github.com/joyent/node/pull/25569

PR-URL: https://github.com/nodejs/node/pull/2843


Reviewed-By: rvagg - Rod Vagg <rod@vagg.org>
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>