* Makefile.in (build_tooldir): new variable, same as old
$(tooldir), but without depending on $(libdir)/$(unlibsubdir)
(GCC_FOR_TARGET): add -B$(build_tooldir)/bin/
(bootstrap, bootstrap2, bootstrap3, bootstrap4): ditto
* configure.in (gxx_include_dir): set default based on unlibsubdir
* Makefile.in (tooldir): ditto
(cccp.o, cpplib.o): use unlibsubdir implicitly through
gxx_include_dir, includedir and tooldir
(protoize.o, unprotoize.o): ditto