Handle composite overlay window during drag and drop sessions

Handle composite overlay window during drag and drop sessions

* configure.ac: Test for the composite extension and use it if
available.
* msdos/sed1v2.inp: Update.
* src/Makefile.in (XCOMPOSITE_LIBS, XCOMPOSITE_CFLAGS): New
variables.
(EMACS_CFLAGS, LIBES): Add new libs and cflags.
* src/xterm.c (x_dnd_get_target_window): Look for proxy on
composite overlay window if mapped.
(x_term_init): Test if the composite extension is available.
* src/xterm.h (struct x_display_info): New fields for composite
extension presence.