Repair the MSDOS build following latest changes.

Repair the MSDOS build following latest changes.

 msdos/sedlibmk.inp (am_libgnu_a_OBJECTS): Edit out
 allocator.$(OBJEXT).  Add editing for the new GNULIB_* and
 REPLACE_* variables.
 src/msdos.c (XMenuActivate, XMenuAddSelection): Adjust argument list
 to changes in oldXMenu/XMenu.h from 2011-04-16.
 <menu_help_message, prev_menu_help_message>: Constify.
 (IT_menu_make_room): menu->help_text is now `const char **';
 adjust.
 src/msdos.h (XMenuActivate, XMenuAddSelection): Adjust prototypes
 to changes in oldXMenu/XMenu.h from 2011-04-16.
 (struct XMenu): Declare `help_text' `const char **'.
 src/xfaces.c <Qunspecified>: Make extern again.
 src/syntax.c: Include sys/types.h before including regex.h, as
 required by Posix.