Revert to using --enable-auto-import in Cygwin builds.

Revert to using --enable-auto-import in Cygwin builds.

Disabling auto-import requires that all libraries we use be careful about
declspecs for exported variables; and it seems they aren't.  This means
that Cygwin will not give us useful info about missing PGDLLIMPORT markers;
but it's probably sufficient that MSVC and Mingw builds do.