[RFA/RFT] libffi reorg

[Paolo Bonzini]

> However, I've also wanted a way for libffi to tell client programs
> about its local configury.  For instance, right now in
> libjava/configure.host we have code that knows whether a given
> platform supports the closure API (or libffi at all).  But why should
> we do that, when libffi already knows this?  I wish we could just
> unconditionally include some libffi header and use some exported
> define to decide what to do.  Any thoughts on that?

FFI_CLOSURES is exported (through ffitarget.h) by ffi.h for example.

Paolo