The error message is obvious -funconfigured-libstdc++-v3 is used
on the g++ command line.  So we just add the dependancy.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

ChangeLog:

	PR bootstrap/67102
	* Makefile.def: Have configure-target-libffi depend on
	all-target-libstdc++-v3.
	* Makefile.in: Regenerate.