Add tools/cmake_gen

[mossheim]

Purpose and Motivation

This PR adds scripts that can be used to easily setup a supercollider
server plugin.

Because language plugins are not out of the question in the future, I
have explicitly used the terminology "server plugin" in many places.

These scripts are used in the cookiecutter server plugin repo:
https://github.com/supercollider/cookiecutter-supercollider-plugin

This PR also contains one change to the code in one of the plugin interface headers to eliminate all compiler warnings when compiling the sample plugin.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• All tests are passing
• If necessary, new tests were created to address changes in PR, and tests are passing
• Updated documentation, if necessary
• This PR is ready for review

Since this only adds external tooling, it should be fine going into the
3.10 branch.

You can see an example of the cookiecutter project in action here: https://github.com/brianlheim/simplegain