unused_prefix_warning - bogus?

[DJ Delorie <dj at redhat dot com>]

$ gcc -E -B/foobar/ -B/blah/ -B/garbage/ foo.c > /dev/null
gcc: file path prefix `/garbage/' never used

What about the other two?

Also, configure passes a -B option to gcc to point to the correct
crt0.o to use for building target utilities (cygwin, at least, uses
this).  However, whenever we're not actually linking, gcc emits a
warning which configure mistakes for an error.  This breaks configure,
making it think that it has *no* include files, because all compiles
"fail".

I understand how this warning originated, but given how overused it is
inside gcc (it also causes -L's and -isystem's), perhaps this warning
should now be silent, or enabled under more strict circumstances?  I
can't see how gcc would ever know if the -B's were used by cpp or ld.