libcdecl: Use Bison's api.token.raw feature.

libcdecl: Use Bison's api.token.raw feature.

By default, Bison numbers user-defined tokens starting from 257.
If the scanner never returns plain characters (which is the case
for libcdecl), this is inefficient.

Recent versions of Bison provide the api.token.raw option to improve
things.  Let's turn it on, if configure detects that it is supported.

This requires a very minor tweak to spec_string in order for the
encoding of tokens into single-byte values to work regardless of
which mode Bison is run in.