Use std::regex if possible

It works with gcc >= 4.9.0 and clang (with libc++ or gcc libstdc++ from gcc
>= 4.9.0). The MSVC parg is missing, because I cannot test it, and the
autotools build still link against boost::regex even if it is not needed, but
I don't know how to fix that.
Fix HTML output of \gg and \ll

This is a part of bug #9372. The remaining part (\ne) is not easy to fix,
since \ne is a predefined macro internally, so the HTML output is done for
the expanded macro, and we have no machinery to assign a HTML output to a
complete macro.