Custom package, specifying cmake source directory?

[Matthew Brooks]

I'm working on a custom package definition, but when it gets built the cmake 
files are in "source/src" instead of "source", and so the build system errors 
out when trying to run cmake. How do I specify the source directory for the 
cmake build system?
From grepping around in the definition for the cmake build system, it appears 
that it expects the "source" variable to point to the appropriate location, but 
I'm not at all familiar with scheme or guile, and I have no idea if that's 
correct or how to change it.