* src/assoc.c (assoc_to_string): Use a default: abort (); case

to pacify gcc -Wswitch-default.
* src/scan-gram.l (adjust_location): Use a default: break; case
to pacify gcc -Wswitch-default.
* src/scan-skel.h (skel_get_lineno, skel_get_in, skel_get_out):
(skel_get_leng, skel_get_text, skel_set_lineno, skel_set_in):
(skel_set_out, skel_get_debug, skel_set_debug, skel_lex_destroy):
Move these decls to scan-skel.l, since they don't need to be
visible elsewhere.
* src/scan-skel.l: Accept the above decls.
(skel_lex): New decl, to pacify GCC when unpatched flex 2.5.31
is used.