configure: Do not build the ununsed libffi shared library.

We do not use the shared libffi libraray, nor do we install it.
However, on at least Darwin, the shared version will be picked
up for testing, so it is preferrable not to build it.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

ChangeLog:

	* Makefile.def: Do not build shared libffi.
	* Makefile.in: Regenerate.