[QGIS-Developer] Meaning of GRASS_PREFIX(7)

[Matthew Pickering]

I am attempting to package QGIS for NixOS and I am struggling to
understand what GRASS_PREFIX is specified to do.

When building I pass the `GRASS_PREFIX=...` flag to cmake but the
resulting build does not
propagate the GRASS_PREFIX and I still need to make the grass
executables available on my path.

If I then make the grass binaries available at runtime on my path,
then processing can find the grass tools and the commands work.

Therefore

1. Do I need to pass GRASS_PREFIX at all if QGIS looks for the grass
binary at runtime on my path unconditionally?
2. Is there a way to bake into QGIS the location of the grass binary
at build-time so that I don't also need to provision grass on my PATH
at runtime?
3. What relationship is there between the GRASS plugin and the
processing support for GRASS?

Cheers,

Matt