Small configure.ac refactoring
Change how we handle libffi

I think overall the new approach is simpler. Rather than unpacking
the libffi.a and putting the .o files into our libHSrts.a, we just
use the libffi.a.

This change also means that when compiling programs for the dyn
way, they get explicitly linked against libffi.so (rather than
relying on librts.so being linked against it). This might
fix a problem on FreeBSD, where programs cannot find libffi.so.
Fix the names of the libffi archives

Fixes a bug which meant that they weren't getting installed.