* build-aux/ax_boost_base.m4: Delete.
* build-aux/Makefile.am (dist_m4data_DATA): Remove
ax_boost_base.m4.
* build-aux/vcsn-boost.m4: New file.
* configure.ac: Use VCSN_BOOST instead of AX_BOOST_BASE.  The
latter did too much and we want people to simply set CPPFLAGS
and LDFLAGS if they have an unusual Boost installation root.