It's interesting that when Bruce is talking about using an older, simpler alternative compiler as a "checksum" compiler, the first thing I thought of was pcc[1][2], but it's from the right lab and era that using it might not actually be a good test.

[1] http://pcc.ludd.ltu.se

[2] https://en.wikipedia.org/wiki/Portable_C_Compiler

EDIT: in case you don't follow the links and realize the implications yourself, from wikipedia:

"It was very influential in its day, so much so that at the beginning of the 1980s, the majority of C compilers were based on it."

Also interesting re: reproducible builds: http://lwn.net/Articles/630074/