[DotGNU]libffi changes to pnet

[Rhys Weatherley]

I've updated the CVS version of pnet so that it can
build either with or without libffi.

By default, libffi is turned off on hpux, but turned
on for everything else.  I'll disable other hosts as
we come across ones that don't work.

It is also possible to manually turn off libffi with
the "--without-libffi" option to "configure", but I
wouldn't recommend it unless you really need it.

As stated previously, turning off libffi also turns
off PInvoke support.  There's no way to do PInvoke
in a portable manner without libffi.

I also fixed the hpux/libgc issue in the CVS version,
so it should be possible to get further with the hpux
build now.

Cheers,

Rhys.