Fix duplicate .o file on QNX

* configure.ac (CYGWIN_OBJ): Leave empty on QNX.
Problem reported by Elad Lahav in:
https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00750.html

configure.ac

@@ -5153,8 +5153,6 @@ elif test "$opsys" = "mingw32"; then
   CYGWIN_OBJ=
   PRE_ALLOC_OBJ=
   POST_ALLOC_OBJ=lastfile.o
-elif test "$opsys" = "qnxnto"; then
-  CYGWIN_OBJ=sheap.o
 else
   CYGWIN_OBJ=
   PRE_ALLOC_OBJ=lastfile.o