tools: hide debug-gui help when building with -Ddebug-gui=false

Some distributions, like Fedora, compile libinput with the debug-gui
option set to false.

Running "libinput debug-gui" indicates that the program is not
installed; however, the help message suggests that  the command is
available.

Hide debug-gui from the help message when it is not included.

Fix https://gitlab.freedesktop.org/libinput/libinput/-/issues/480
Signed-off-by: José Expósito <jose.exposito89@gmail.com>

meson.build

@@ -532,6 +532,7 @@ executable('libinput-record',
 	   install : true,
 	   )
 
+config_h.set10('HAVE_DEBUG_GUI', get_option('debug-gui'))
 if get_option('debug-gui')
 	dep_gtk = dependency('gtk4', version : '>= 4.0', required : false)
 	config_h.set10('HAVE_GTK4', dep_gtk.found())

tools/libinput-tool.c

@@ -46,9 +46,11 @@ usage(void)
 	       "  debug-events\n"
 	       "	Print events to stdout\n"
 	       "\n"
+#if HAVE_DEBUG_GUI
 	       "  debug-gui\n"
 	       "	Display a simple GUI to visualize libinput's events.\n"
 	       "\n"
+#endif
 	       "  measure <feature>\n"
 	       "	Measure various device properties. See the man page for more info\n"
 	       "\n"