ignores: update
boost.m4: provide a means to look for different library base names

A recent change seems to be dictated by the fact that on MingW, the
thread library is actually name thread_win32.  The patch is somewhat
uglified because the library name is also used to compute the name of
the output variables.  This shows that it is desirable to separate the
component name (e.g., thread), from the real shared-object base name
(e.g., thread or thread_win32).

* build-aux/boost.m4 (_BOOST_FIND_LIBS): New.
Includes the part of BOOST_FIND_LIB that used to look for the library,
inside the AC_CACHE_CHECK.
(BOOST_FIND_LIBS): New.
Contains what used to be around the library look up.
(BOOST_FIND_LIB): Backward compatibility layer.
boost.m4: use BOOST_FIND_LIBS to deal with thread_win32

* build-aux/boost.m4: Here.
This reduces the generated code bloat.
boost.m4: promote BOOST_THREAD over BOOST_THREADS

* build-aux/boost.m4 (BOOST_THREADS): Rename as...
(BOOST_THREAD): this.
(BOOST_THREADS): New backward compatibility definition.
boost.m4: beware that Boost.Python may have a libboost_python3 library

This is the case on ArchLinux for instance.

* build-aux/boost.m4 (BOOST_PYTHON): Check the possible python3
library basename.
* README, tests/testsuite.at: Adjust.
boost.m4: fix the witness header for Boost.LogSetup

* build-aux/boost.m4: I don't have boost/log/utility/init/from_settings.hpp,
but boost/log/utility/setup/from_settings.hpp.
I have no idea when this was changed, and no desire to know :)
boost.m4: style changes

* build-aux/boost.m4: Don't call AC_SUBST gratuitously.
Indent.
Comment changes.
boost.m4: introduce Boost.Flyweight check

* build-aux/boost.m4 (_BOOST_PTHREAD_FLAG): Documentation changes.
(BOOST_FLYWEIGHT): New.