curses-compat: Improve probe for request_mouse_pos.

On AIX 7.2, the request_mouse_pos function is defined but it seems
the related macros (e.g., MOUSE_X_POS) don't actually work, leading to
undefined variable references at link time.  The configure test should
have caught this, but it turns out GCC optimizes out the check.

So configure concludes request_mouse_pos is supported and then later
compilation of programs which use this feature fail to link.  Ensure
the macro usage influences the program return value so GCC doesn't
delete it.

diff --git a/m4/curses-compat.m4 b/m4/curses-compat.m4
index c445a67f2366740ea619fa029af5c62f5b6b530b..c24a5ad9563b44b2c9bb9945c55101f6855abd61 100644 (file)
--- a/m4/curses-compat.m4
+++ b/m4/curses-compat.m4
@@ -1,4 +1,4 @@
-dnl Copyright © 2022 Nick Bowler
+dnl Copyright © 2022, 2024 Nick Bowler
 dnl
 dnl Macros for probing specific curses library behaviour.
 dnl
@@ -68,7 +68,7 @@ AS_CASE([$dx_cv_curses_have_mouse_set$dx_cv_curses_have_mousemask],
 DX_CHECK_CURSES_GETMOUSE_NCURSES
 AS_IF([test x"$dx_cv_curses_have_getmouse_ncurses" != x"yes"],
   [DX_CHECK_CURSES_FUNC([request_mouse_pos], [],
-    [switch (BUTTON_STATUS(1)) case BUTTON_PRESSED:;])])
+    [switch (BUTTON_STATUS(1)) case BUTTON_PRESSED: return 42;])])
 AS_CASE(
 [$dx_cv_curses_have_getmouse_ncurses$dx_cv_curses_have_request_mouse_pos],
   [*yes*], [AC_DEFINE([HAVE_CURSES_MOUSE_SUPPORT], [1],