[Bug bootstrap/57125] Build not SMP safe; fails to build bconfig.h

[andrew at ado dot is-a-geek.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57125

--- Comment #6 from Andrew Oakley <andrew at ado dot is-a-geek.net> ---
Sorry, this patch doesn't seem sufficient (perhaps I was just lucky for a
while).  Apologies for the noise.