patch 8.0.0949: winpty.dll name is fixed

Problem:    winpty.dll name is fixed.
Solution:   Add the 'winptydll' option. Make the default name depend on
            whether it is a 32-bit or 64-bit build. (idea by Yasuhiro
            Matsumoto, closes #1978)
diff --git a/src/option.h b/src/option.h
index 9b7e88f..b772a8e 100644
--- a/src/option.h
+++ b/src/option.h
@@ -966,6 +966,9 @@
 EXTERN long	p_wmw;		/* 'winminwidth' */
 EXTERN long	p_wiw;		/* 'winwidth' */
 #endif
+#if defined(WIN3264) && defined(TERMINAL)
+EXTERN char_u	*p_winptydll;	/* 'winptydll' */
+#endif
 EXTERN int	p_ws;		/* 'wrapscan' */
 EXTERN int	p_write;	/* 'write' */
 EXTERN int	p_wa;		/* 'writeany' */