[Bug bootstrap/81926] go/parse.o differs between stage2 and stage3

[dclarke at blastwave dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81926

--- Comment #25 from Dennis Clarke <dclarke at blastwave dot org> ---
So this seems to be related to some env var BUILD_CONFIG ? 

Ehere is this "BUILD_CONFIG" documented?