tiny winsup config patch

[DJ Delorie]

> I don't think that it should be the function of winsup's configure to
> ensure that the build environment which surrounds it is ok.  Cygnus
> provides a build environment.  If you don't want to use it, then you
> take your lumps.

I think winsup's configure needs to know if it's building with a local
version of newlib et al, and use it directly if so, rather that use
newlib's libc.a (via libcygwin.a) but the installed cross compiler's
headers.

> If you're going to check for ../newlib, then you should check for
> ../libiberty and ../include, too.  Do we really want to go down that
> path?

For building winsup and winsup/utils, I think we must.  That's the
library we link against (via libcygwin.a), those should be the headers
we include.