$prefix/lib/../$target/sys-include not in <> search path

[Hans-Peter Nilsson]

When configured with just a --prefix=x and --target=y,
$prefix/lib/../$target/sys-include used to be searched, for e.g.
limits.h, stdio.h and stdlib.h.  No $prefix-rooted path shows
up as a "ignoring nonexistent directory" message either.

I don't know when this changed, but it doesn't seem like a
deliberate change.  Or at least, not an improvement.

I'll open a PR if I can't resolve this.

brgds, H-P