Add support for basic syncing with the window manager on resize

Add support for basic syncing with the window manager on resize

This is handled by GTK 3, so the code is disabled on that
specific build.  On other builds, this eliminates any unexposed
part of a frame from showing up after a resize when
`frame-resize-pixelwise' is t.

* configure.ac: Check for the X Synchronization Extension if
present.
* src/Makefile.in (EMACS_CFLAGS):
(LIBES): Add XSYNC_LIBS and XSYNC_CFLAGS.

* src/xfns.c (append_wm_protocols): Declare
`_NET_WM_SYNC_REQUEST' support if appropriate.
(x_window): Adjust location of call to `append_wm_protocols' on
Xt version to prevent it from being overwritten.
(Fx_create_frame): Create basic counter.
* src/xterm.c (XTframe_up_to_date): Set counter value to the one
asked for by the window manager.
(handle_one_xevent): Handle _NET_WM_SYNC_REQUEST.
(x_free_frame_resources): Free frame counter if present.
(x_term_init): Test for XSync and set fields accordingly.
* src/xterm.h (struct x_display_info): New fields for XSync
support and new atoms.
(struct x_output): New fields for counter status.
(FRAME_X_BASIC_COUNTER): New macro.