meson: only build demo manpages when demos are enabled

Installing them even when manpages are enabled, but the demos are
disabled, produces manpages for unavailable programs...

docs/reference/gtk/meson.build

@@ -70,16 +70,21 @@ if get_option('man-pages') and xsltproc.found()
   man_files = [
     [ 'gtk4-broadwayd', '1', ],
     [ 'gtk4-builder-tool', '1', ],
-    [ 'gtk4-demo', '1', ],
-    [ 'gtk4-demo-application', '1', ],
     [ 'gtk4-encode-symbolic-svg', '1', ],
-    [ 'gtk4-icon-browser', '1', ],
     [ 'gtk4-launch', '1', ],
     [ 'gtk4-query-settings', '1', ],
     [ 'gtk4-update-icon-cache', '1', ],
-    [ 'gtk4-widget-factory', '1', ],
   ]
 
+  if get_option('demos')
+    man_files += [
+      [ 'gtk4-demo', '1', ],
+      [ 'gtk4-demo-application', '1', ],
+      [ 'gtk4-widget-factory', '1', ],
+      [ 'gtk4-icon-browser', '1', ],
+    ]
+  endif
+
   foreach man: man_files
     man_name = man.get(0)
     man_section = man.get(1, '1')