Import new boost M4 files from upstream

    * m4/ax_boost_base.m4: Imported from autoconf-archive-2014.02.28,
    * m4/ax_boost_test_exec_monitor.m4: Ditto,
    * m4/ax_boost_unit_test_framework.m4: Ditto.

Signed-off-by: Dodji Seketeli <dodji@seketeli.org>
Use BOOSTLIBDIR instead of harcoded path to find boost

    * configure.ac: Use BOOSTLIBDIR variable to find libboost.

Signed-off-by: Dodji Seketeli <dodji@seketeli.org>