Moving an installed gcc/glibc crosstoolchain to a different directory was not allowed for gcc-2.96 and below, I seem to recall, but became permissible around gcc-3.0.
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.
As for how this interracts with PCH, that I can't answer. I haven't tried it.
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Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
