Remove --enable-auto-import linker switch in Cygwin build.

This is expected to make it start failing when contrib modules
reference non-PGDLLIMPORT'ed global variables, as the other Windows
build methods do.  Aside from the value of consistency, the underlying
implementation of this switch is pretty ugly and not really something
we want to rely on if we have to use PGDLLIMPORT anyway for MSVC.