When compiling Gtk+...
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_visited'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_remove_item'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_hash_table_get_type'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_intern_static_string'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_object_force_floating'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_pdf_surface_create'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_description'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_pdf_surface_create_for_stream'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_key_file_get_double'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_modified'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_has_item'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_is_private'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_add_group'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_app_info'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_title'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_object_is_floating'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_groups'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_load_from_file'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_key_file_set_double'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_uris'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_free'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_hash_table_ref'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_set_description'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_slice_alloc'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_applications'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_object_ref_sink'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_set_title'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_initially_unowned_get_type'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_add_application'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_added'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_type_register_static_simple'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_param_spec_ref_sink'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_size'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_to_file'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_new'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_hash_table_unref'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_surface_set_fallback_resolution'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_set_is_private'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_move_item'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_slice_free_chain_with_offset'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_intern_string'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_get_mime_type'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_pdf_surface_set_size'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_bookmark_file_set_mime_type'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_surface_get_type'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_date_set_time_t'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_slice_free1'
./.libs/libgtk-x11-2.0.so: undefined reference to `cairo_ps_surface_set_size'
./.libs/libgtk-x11-2.0.so: undefined reference to `g_slice_alloc0'
I'm guessing Slack 12.0 doesn't come with the X headers, eh? Where can I get them, or if I'm wrong, what happened?