build: check if libinput supports hold gestures

Add a project argument (LIBINPUT_HAS_HOLD_GESTURES) to allow building
against old versions of libinput.

backend/libinput/meson.build

@@ -27,3 +27,9 @@ wlr_files += files(
 
 features += { 'libinput-backend': true }
 wlr_deps += libinput
+
+# libinput hold gestures are available since 1.19.0
+add_project_arguments(
+	'-DLIBINPUT_HAS_HOLD_GESTURES=@0@'.format(libinput.version().version_compare('>=1.19.0').to_int()),
+	language: 'c',
+)