Allow use of pw-jack with custom variable

[noonker]

I'm using PipeWire under Linux and need to use pw-jack to start sclang to ensure that we're using PipeWire's implementation of Jack instead of Jack's own implementation.

This patch adds the custom variable

sclang-use-pw-jack. When it's non-nil- sclang-start will load pw-jack instead of sclang-program and instead add sclang-program to the first argument generated by sclang-make-options

Let me know what you think :)