- [GUI] prevented (using a workaround) a crash when an external program can not be found.

configure.ac

@@ -161,6 +161,13 @@ if test "$WISH" = "no" ; then
    exit 1;
 fi
 
+AC_PATH_PROGS([WHICH],[which],[no])
+
+if test "$WHICH" = "no" ; then
+   AC_MSG_ERROR([Can not find which executable.])
+   exit 1;
+fi
+
 AC_PATH_PROGS([SDL2_CONFIG],[sdl2-config],[no])
 
 if test "$SDL2_CONFIG" = "no" ; then