Boost with the ENABLE trick.

[[Imported from SVN: r6693]]

cmake/modules/DuneBoost.cmake

+find_package(Boost)
+
+message("Boost_LIBRARIES=${Boost_LIBRARIES}")
+message("Boost_INCLUDE_DIRS=${Boost_INCLUDE_DIRS}")
+
+function(add_dune_boost_flags _targets)
+  if(Boost_FOUND)
+    include_directories("${Boost_INCLUDE_DIRS}")
+    foreach(_target ${_targets})
+      target_link_libraries(${_target} ${Boost_LIBRARIES})
+      # The definitions are a hack as we do not seem to know which MPI implementation was
+      # found.
+      GET_TARGET_PROPERTY(_props ${_target} COMPILE_FLAGS)
+      string(REPLACE "_props-NOTFOUND" "" _props "${_props}")
+      SET_TARGET_PROPERTIES(${_target} PROPERTIES COMPILE_FLAGS
+	"${_props} -DENABLE_BOOST=1")
+    endforeach(_target)
+  endif(Boost_FOUND)
+endfunction(add_dune_boost_flags)