Make mate_gdk_spawn_command_line_on_screen available in GTK2 build

This patch enables mate_gdk_spawn_command_line_on_screen in GTK2 build as the original gdk_spawn_command_line_on_screen function has been deprecated in GDK 2.24 and causes compiler warnings in application code.

Also the patch improves the replacement function to avoid a GTK3 deprecation warning, adds a missing newline in configure summary output, changes #ifdef __cplusplus in libmate-desktop to G_{BEGIN,END}_DECLS and removes trailing newlines in mate-desktop-utils.c