Boost is updated to a more recent version to avoid many compilation
warnings. However, boost 1.76 introduces a brand new regex library.
Therefore, it was chosen to use the last version before that to
minimize changes.
The part that adapts our source to the new boost requirements is
backported from 09130d7a62.
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.
