Fix MSDOS build broken by autoconfiscation and changes in symbol handling.

Fix MSDOS build broken by autoconfiscation and changes in symbol handling.

 src/msdos.c (dos_set_window_size):
 src/w16select.c (Fx_selection_exists_p): Use `Fsymbol_value (foo)'
 instead of `XSYMBOL (foo)->value'.
 config.bat: Allow for 2 leading `#'s in comments in
 src/Makefile.in.
 src/Makefile.in: Don't use Make-style comments past the "start of
 cpp stuff" line.
 (MSDOS_OBJ): Remove xmenu.o (it is now defined by XMENU_OBJ).
 src/s/msdos.h (UNEXEC): Don't define (@unexec@ in Makefile.in is
 edited directly by msdos/sed1v2.inp).
 msdos/sed3v2.inp (C_SWITCH_MACHINE): Edit to empty.
 msdos/sed1v2.inp (@LIB_MATH@): Edit to -lm.
 (C_SWITCH_MACHINE, C_SWITCH_SYSTEM, LD_SWITCH_SYSTEM_TEMACS)
 (LD_SWITCH_X_SITE_AUX, LD_SWITCH_X_SITE_AUX_RPATH)
 (LD_SWITCH_SYSTEM, LD_SWITCH_SYSTEM_EXTRA, LIBTIFF, LIBJPEG)
 (LIBPNG, LIBGIF, LIBXPM, XFT_LIBS, DBUS_CFLAGS, DBUS_LIBS)
 (DBUS_OBJ, GCONF_CFLAGS, GCONF_LIBS, GTK_OBJ, LIBXMU, LIBXSM)
 (LIBXTR6, XOBJ, TOOLKIT_LIBW, WIDGET_OBJ, CYGWIN_OBJ, NS_OBJ)
 (NS_SUPPORT, LIBSELINUX_LIBS, START_FILES): Edit to empty.
 (XMENU_OBJ): Edit to xmenu.o.
 (FONT_OBJ): Edit to value used for X on Unix.
 (PRE_ALLOC_OBJ): Edit to lastfile.o.
 (POST_ALLOC_OBJ): Edit to $(vmlimitobj).
 (@unexec@): Edit to unexec.o.
 lisp/cus-start.el (all): Add native condition for tool-bar-* symbols.