As 'which imake' is returning an empty result in a minimal buildroot, such as one mock creates, rpm sees an empty BuildRequire, making it bail. Attached is a patch, which changes the %{imake} dependency to BuildRequire: imake. This makes transfig compile in mock.
Created attachment 129139 [details] Patch, fixing the imake dependency
*** Bug 191717 has been marked as a duplicate of this bug. ***
it's now fixed in rawhide. Thanks for youre report.