`XCC= <relative cc> -B<path to relative cc1> ...'. This is equivalent
when cc and cc1, etc. have just been bootstrapped by `make world'.
The relative versions normally won't work if the target system is
not binary compatible. Bootstrapping different versions of gcc
without going through `make world' is slightly more broken than
before.
Uniformized macro names (P1OBJS -> LIB1POBJS, etc.).
Don't give full paths to sources.