* Michael Stapelberg:
Interesting. Do you have a pointer to an example (and/or more documentation) for this happening in the wild?
glibc has architecture-specific build dependencies.
However, many packages use non-trivial things for unpacking the sources (the %prep stage), and you have to install the build depencies before you can complete the %prep stage. I have written a mock wrapper to do this, but it's not completely reliable due to dangling symbolic links: