parser: add some notes about byacc working

parser: add some notes about byacc working

We now also work with byacc (version tested: 20130925) which some people
prefer, perhaps due to its license (public domain) or performance
(haven't compared).

When using byacc, currently the following warning comes up:
    src/xkbcomp/parser.c:954:14: warning: declaration shadows a variable in the global scope [-Wshadow]
        YYSTYPE  yylval;
                ^
    src/xkbcomp/parser.c:37:20: note: expanded from macro 'yylval'
    #define yylval     _xkbcommon_lval
                    ^
    ./src/xkbcomp/parser.h:96:16: note: previous declaration is here
    extern YYSTYPE _xkbcommon_lval;

This is due to a bug in byacc - it shouldn't output that extern line in
%pure-parser mode. So the warning stays.

Signed-off-by: Ran Benita <ran234@gmail.com>