Suppress signed/unsigned comparison warnings for yycheck.

* data/c.m4 (b4_safest_int_type): New macro.
* data/glr.c, data/lalr1.cc: Wherever you compare yycheck[i] against
a signed int type, cast it to b4_safest_int_type first.
* data/yacc.c: Likewise.
(b4_safest_int_type): Overwrite the one from c.m4 since b4_int_type is
also overwritten.