Cygwin: configure: Add option to disable building 'dumper'

Cygwin: configure: Add option to disable building 'dumper'

Rather than guessing, based on just the presence of libbfd, add an
explicit configuration option, to build dumper or not, defaulting to
building it.

This might have some use when bootstrapping Cygwin for a new
architecture, or when building your own Cygwin-targetted cross-compiler,
rather than installing one from the copr, along with the dependencies of
libbfd.