yacc.c: factor.

yacc.c: factor.

yacc.c used to include two almost identical sections: one for the *.h
file, and another for the *.c file.  The main difference is that in
the *.c file we used the yy* names (as %name-prefix is handled by
"#define yy* <prefix>*" before), while the *.hh used <prefix>* names.
Keep only the later.  If this is troublesome, b4_shared_declarations
can easily take the desired prefix as argument.

* data/yacc.c (b4_shared_declarations): New.
Use it to factor duplicated declarations.