Fix the Haiku port

* src/haiku_support.h [__cplusplus]: Define `specpdl_ref' type
on C++ since that code cannot include lisp.h.

src/haiku_support.h

@@ -348,6 +348,24 @@ struct haiku_menu_bar_state_event
 #define BE_RECT_WIDTH(rect) (ceil (((rect).right - (rect).left) + 1))
 #endif /* __cplusplus */
 
+/* C++ code cannot include lisp.h, but file dialogs need to be able
+   to bind to the specpdl and handle quitting correctly.  */
+
+#ifdef __cplusplus
+
+#if SIZE_MAX > 0xffffffff
+#define WRAP_SPECPDL_REF 1
+#endif
+#ifdef WRAP_SPECPDL_REF
+typedef struct { ptrdiff_t bytes; } specpdl_ref;
+#else
+typedef ptrdiff_t specpdl_ref;
+#endif
+
+#else
+#include "lisp.h"
+#endif
+
 #ifdef __cplusplus
 extern "C"
 {