patch 8.1.2171: mouse support not always available

Problem:    Mouse support not always available.
Solution:   Enable mouse support also in tiny version.  Do not define
            FEAT_MOUSE_XTERM on MS-Windows (didn't really work).
diff --git a/src/feature.h b/src/feature.h
index b2ba162..04bb30c 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -964,12 +964,12 @@
  *			console mouse handling.
  * +mouse_urxvt		Unix only: Include code for for urxvt mosue handling.
  * +mouse		Any mouse support (any of the above enabled).
+ *			Always included, since either FEAT_MOUSE_XTERM or
+ *			DOS_MOUSE is defined.
  */
 /* OS/2 and Amiga console have no mouse support */
-#if !defined(AMIGA)
-# ifdef FEAT_NORMAL
-#  define FEAT_MOUSE_XTERM
-# endif
+#if defined(UNIX) || defined(VMS)
+# define FEAT_MOUSE_XTERM
 # ifdef FEAT_BIG
 #  define FEAT_MOUSE_NET
 # endif
@@ -979,12 +979,12 @@
 # ifdef FEAT_BIG
 #  define FEAT_MOUSE_URXVT
 # endif
-# if defined(FEAT_NORMAL) && defined(MSWIN)
-#  define DOS_MOUSE
-# endif
-# if defined(FEAT_NORMAL) && defined(__QNX__)
-#  define FEAT_MOUSE_PTERM
-# endif
+#endif
+#if defined(MSWIN)
+# define DOS_MOUSE
+#endif
+#if defined(__QNX__)
+# define FEAT_MOUSE_PTERM
 #endif
 
 /*
@@ -1002,28 +1002,11 @@
 # define FEAT_SYSMOUSE
 #endif
 
-/* urxvt is a small variation of mouse_xterm, and shares its code */
+// urxvt is a small variation of mouse_xterm, and shares its code
 #if defined(FEAT_MOUSE_URXVT) && !defined(FEAT_MOUSE_XTERM)
 # define FEAT_MOUSE_XTERM
 #endif
 
-/* Define FEAT_MOUSE when any of the above is defined or FEAT_GUI. */
-#if !defined(FEAT_MOUSE_TTY) \
-	&& (defined(FEAT_MOUSE_XTERM) \
-	    || defined(FEAT_MOUSE_NET) \
-	    || defined(FEAT_MOUSE_DEC) \
-	    || defined(DOS_MOUSE) \
-	    || defined(FEAT_MOUSE_GPM) \
-	    || defined(FEAT_MOUSE_JSB) \
-	    || defined(FEAT_MOUSE_PTERM) \
-	    || defined(FEAT_SYSMOUSE) \
-	    || defined(FEAT_MOUSE_URXVT))
-# define FEAT_MOUSE_TTY		/* include non-GUI mouse support */
-#endif
-#if !defined(FEAT_MOUSE) && (defined(FEAT_MOUSE_TTY) || defined(FEAT_GUI))
-# define FEAT_MOUSE		/* include generic mouse support */
-#endif
-
 /*
  * +clipboard		Clipboard support.  Always used for the GUI.
  * +xterm_clipboard	Unix only: Include code for handling the clipboard
@@ -1093,7 +1076,7 @@
  *			to check if mouse dragging can be used and if term
  *			codes can be obtained.
  */
-#if (defined(FEAT_NORMAL) || defined(FEAT_MOUSE)) && defined(HAVE_TGETENT)
+#if defined(HAVE_TGETENT)
 # define FEAT_TERMRESPONSE
 #endif