* src/pgtkselect.c (pgtk_reply_selection_request): Remove another
unused variable.

* src/xsettings.c (store_tool_bar_style_changed): Compile only
under HAVE_GSETTINGS.
(xg_get_system_dark_mode): Compile only under HAVE_GSETTINGS.
* src/gtkutil.c (xg_set_gtk_theme_dark_mode)
(xg_update_dark_mode_for_all_displays, xg_set_initial_dark_mode):
Only compile when HAVE_GSETTINGS is defined.
(xg_create_frame_widgets): Call 'xg_set_initial_dark_mode' only
under HAVE_GSETTINGS.
(xg_create_frame_outer_widgets): Remove unused code.  Call
'xg_set_initial_dark_mode' only under HAVE_GSETTINGS.
* src/pgtkselect.c (pgtk_reply_selection_request): Remove unused
variable.

* src/pgtkfns.c:
* src/pgtkim.c:
* src/pgtkmenu.c:
* src/pgtkselect.c:
* src/pgtkterm.c:
* src/termcap.c:
* src/tparam.c: Delete superfluous comments explaining config.h.

This is a better fix for Bug#76414.
* src/pgtkselect.c (pgtk_nalloc): New function.
(pgtk_get_window_property, lisp_data_to_selection_data): Use it.