simplify boost use script, remove Boost_LIBS
simplify boost use script, remove Boost_LIBS

no reason to keep Boost_LIBS around any longer
now, more simple
  xpFindPkg(PKGS boost)
  target_link_libraries(foo PRIVATE Boost::iostreams)
instead of more complicated
  set(Boost_LIBS iostreams)
  xpFindPkg(PKGS boost)
  target_link_libraries(foo PRIVATE ${BOOST_LIBRARIES})
the cmake boost targets have cleaned-up boost a lot!

issue https://github.com/smanders/externpro/issues/308

smanders/externpro