Installer file: use macros to ease values replacement for a new codec

[rotoglup]

Hello,

I've been using your example to try to create a new codec, all went smooth, thanks for this !

Along the way, I had most of my issues with the registry values, so I thought it would be helpful to use macros to identify and define which values are supposed to be changed in the install script.

FWIW, you'll find what I came up with when changing values for my derived codec.