Give up on -Wsuggest-attribute=const

Give up on -Wsuggest-attribute=const

The attribute doesn't help performance significantly, and the
warning seems to be more trouble than it's worth.  See the thread at:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00361.html
* configure.ac (WERROR_CFLAGS): Don't use -Wsuggest-attribute=const.
* lib-src/make-docfile.c (write_globals):
Remove special hack for Fnext_read_file_uses_dialog_p.
* src/decompress.c (Fzlib_available_p):
* src/gnutls.c (Fgnutls_available_p):
* src/gtkutil.h (xg_uses_old_file_dialog):
* src/xdisp.c (Ftool_bar_height):
* src/xmenu.c (popup_activated):
No longer const, since it's not const on at lest some
configurations, and we shouldn't lie to the compiler.