Blind attempt at a Cygwin fix

Further portability fix for a967613911.  Mingw- and MSVC-based builds
appear to be working fine, but Cygwin needs an extra tweak whereby the
new win32security.c file is explicitely added to the list of files to
build in pgport, per Cygwin members brolga and lorikeet.

Author: Michael Paquier

configure

@@ -13075,6 +13075,12 @@ if test "$PORTNAME" = "cygwin"; then
  ;;
 esac
 
+  case " $LIBOBJS " in
+  *" win32security.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS win32security.$ac_objext"
+ ;;
+esac
+
 fi
 
 ac_fn_c_check_decl "$LINENO" "sys_siglist" "ac_cv_have_decl_sys_siglist" "#include <signal.h>

configure.in

@@ -1596,6 +1596,7 @@ fi
 # Cygwin needs only a bit of that
 if test "$PORTNAME" = "cygwin"; then
   AC_LIBOBJ(dirmod)
+  AC_LIBOBJ(win32security)
 fi
 
 AC_CHECK_DECLS([sys_siglist], [], [],