[cmake] correctly register the found boost libs

The old behavior only added `-lboost_system` but did not add
`-L/path/to/boost/lib`.

cmake/modules/DuneXtCommonMacros.cmake

@@ -14,8 +14,11 @@ include(XtTooling)
 set(DS_REQUIRED_BOOST_LIBS system thread filesystem date_time timer chrono)
 FIND_PACKAGE(PkgConfig)
 FIND_PACKAGE(Boost 1.48.0 COMPONENTS ${DS_REQUIRED_BOOST_LIBS} REQUIRED)
+dune_register_package_flags(INCLUDE_DIRS ${Boost_INCLUDE_DIRS})
 foreach(_boost_lib ${DS_REQUIRED_BOOST_LIBS})
-    dune_register_package_flags(LIBRARIES boost_${_boost_lib})
+  set(_BOOST_LIB "")
+  string(TOUPPER "${_boost_lib}" _BOOST_LIB)
+  dune_register_package_flags(LIBRARIES ${Boost_${_BOOST_LIB}_LIBRARY})
 endforeach(_boost_lib ${DS_REQUIRED_BOOST_LIBS})
 
 find_package(Eigen3 3.2.0)