Fixed compilation warning for clang.

Sample warning:
    warning: struct 'RegExp' was previously declared as a class [-Wmismatched-tags]
    struct RegExp; // forward
    ^
Got rid of asciidoc build-time dependency.
Release 0.13.7.3.
Update version to 0.13.7.4.dev
release.sh: clean tarball directory before rsync'ing to sourceforge

(shouldn't push old tarballs)
run_tests.sh: avoid non-portable sed feature
Fixed compile error for freebsd5 (found by Sergei Trofimovich).

Sample error:
    parser.y: In function `void re2c::parse(re2c::Scanner&, std::ostream&, std::ostream*)':
    parser.y:564: error: `yyparse' undeclared (first use this function)
Renamed configure.in -> configure.ac
Disallow to use yacc/byacc instead of bison to build parser.

Currently parser fails to build with yacc/byacc.
A slightly modified parser, build by byacc, causes 3 tests to fail.