Change Working Directory for a compile task?

Is it possible to change the “Working directory” for a forked compile ?

I have a multi project build and use settings,gradle to define the tops of each of the subprojects:
project(":daves").projectDir = file(“modules/daves”)

The problem I am encountering is when I try the new JDK9 modules which requires a declared modulesourcepath which is used to grok the module name to be compiled. Due to an implementation detail, the javac compiler assumes the modulesourcepath is relative to current working directory, which in my case means I have lost the module name (“daves”).

So I have to rework my source trees to add another level with the module name redundantly in, or I need to figure out how to change the working directory for the compile invocation, or perhaps get a clue for a different way around the issue.