Moving trees around has worked for a long time, but it required
manually setting the GCC_EXEC_PREFIX environment variable.
Cygnus got this working reliably sometime in the early '90s I think.
In gcc-3.0 and later, there is code (make_relative_prefix) that
computes and sets GCC_EXEC_PREFIX for you, so now you no longer
need to manually set an environment variable to make it work.
It just works by default.
Once GCC_EXEC_PREFIX is set, it overrides pretty much all of the
builtin paths, and since GCC_EXEC_PREFIX is always set, either
explicitly by the user, or implicitly by gcc, the builtin paths should be mostly irrelevant.