Cygwin build fixes.

Get rid of use of dlltool for linking the main postgres executable.
dlltool is obsolete and we'd prefer to stop depending on it.

Also, include $(LDAP_LIBS_FE) in $(libpq_pgport).  (It's not clear that
this is really needed, or why it's not a linker bug if it is needed.
But reports are that it's needed on current Cygwin.)

We might want to back-patch this if it works, but first let's see
what the buildfarm thinks.

Marco Atzeri