Do not override ADD_LIBS or COMPILE_FLAGS during expansion.

[[Imported from SVN: r6998]]

cmake/modules/DuneMacros.cmake

@@ -614,8 +614,8 @@ endmacro(dune_common_script_source_dir)
 
 function(dune_expand_object_libraries _SOURCES_var _ADD_LIBS_var _COMPILE_FLAGS_var)
   set(_new_SOURCES "")
-  set(_new_ADD_LIBS "")
-  set(_new_COMPILE_FLAGS "")
+  set(_new_ADD_LIBS "${${_ADD_LIBS_var}}")
+  set(_new_COMPILE_FLAGS "${${_COMPILE_FLAGS_var}}")
   set(_regex "_DUNE_TARGET_OBJECTS:([a-zA-Z0-9_-]+)_")
   foreach(_source ${${_SOURCES_var}})
     string(REGEX MATCH ${_regex} _matched "${_source}")