t/callext.t - include the pdlsimple.h that's in blib.
Previously Basic/Core/pdlsimple.h was being included.
It should be the same as blib/lib/PDL/Core/pdlsimple.h,
but it's the *latter* that we really should be testing.
I think this is a portable solution. If I'm wrong about
that, I'm sure we'll soon find out. (We would then
probably have to revert to including the former.)