cmake: use keyword signature of target_link_libraries

cmake/modules/AddIPOptFlags.cmake

@@ -7,7 +7,7 @@ function(add_dune_ipopt_flags _targets)
       set_target_properties(${_target} PROPERTIES COMPILE_FLAGS
           "${_props} ${DUNE_IPOPT_CFLAGS}")
       if(NOT ADD_DUNE_IPOPT_FLAGS_OBJECT)
-        target_link_libraries(${_target} ${IPOPT_LIBRARY})
+        target_link_libraries(${_target} PUBLIC ${IPOPT_LIBRARY})
       endif()
     endforeach(_target ${_targets})
   endif(IPOPT_FOUND)