Prefix for file in source directory used in ./configure?

[Jason Sewall]

Hello,

I've searched the docs and the web for info on this, but I can't seem
to make headway.

I have a script in the same directory as my configure.ac that tries to
clean what commit in my VC we're building.

GIT_VERSION=`./GIT-VERSION-GEN`

Obviously, that './' in there doesn't work if I try to configure
outside my source directory. What variable do I need add as prefix to
that path to let builds outside the source tree find that script?

Cheers,
Jason