installer: automatically embed VCRedist 12.0 DLLs when env var MumbleNoMergeModule is set.

This commit updates the old RedistDirVC10 files to point to their new version 12 copies,
now that we build with MSVS2013.

It also implements an easy way to opt-out of using merge modules for the VCRT
redistributable DLLs.

With this change, if the environment variable MumbleNoMergeModule is set,
the installer build will automatically embed the VCRedist DLL files as
regular files into the final .MSI.

To avoid this behavior, one can set MumbleNoEmbedVCRedist=1. So, to build
a Mumble installer with no VCRT merge modules *and* no embedded VCRT DLLs,
one would set *both*

  MumbleNoMergeModules=1
  MumbleNoEmbedVCRedist=1