passing macros into mock

[Tim Jackson]

Is there any way (not in the live buildsys, but locally for testing 
purposes) to pass a macro that isn't normally defined into a mock build 
environment? I was hoping to do something like:

mock --define "something 123" blah.src.rpm

I had a search around but couldn't find any obvious answers. Any easy 
way short of rebuilding the SRPM outside the mock environment with the 
necessary macro defined?

TIA,

Tim