This requires to add a assertion_failed_msg handler. Take this
occasion to cleanup and uniformize the various boost.cpp files that we
have.
This adds 50k line of whatever to the source code.
icu.{hpp,cpp} is not needed and cannot be compiled since it depends on libicu.
The other two files are currently not used, but could be in principle, so I
added them to the build.
Boost.Signals is deprecated. This fixes bug #9943.
The only thing left to do is to rewrite (or get rid of) the boost -mt test
in config/lyxinclude.m4 not to use signals anymore.
