On May 21, Lucas De Marchi [off-list ref] wrote:

And how is this signature prepared?  Since it needs the compiled
module it would be a matter of changing the compiler, even minor
version, to invalidate the argument of reproducible build. It seems
very fragile to me.

But this is the whole point of reproducible builds: knowing that if you 
start from the same sources and build enviroment, and distributions do, 
then you will get the same results.
Building gcc is reproducibile as well...
Reproducibility with a different compiler has never been a goal.

-- 
ciao,
Marco