Automatically add IpOpt flags if dune_enable_all_flags is used

cmake/modules/FindIPOpt.cmake

@@ -37,4 +37,9 @@ if(IPOPT_FOUND)
     if (DL_FOUND)
         set (IPOPT_LIBRARY ${DL_LIBRARY} ${IPOPT_LIBRARY})
     endif(DL_FOUND)
+
+    # register all related flags
+    dune_register_package_flags(COMPILE_DEFINITIONS "ENABLE_IPOPT=1"
+                                LIBRARIES "${IPOPT_LIBRARY}"
+                                INCLUDE_DIRS "${IPOPT_INCLUDE_DIR}")
 endif(IPOPT_FOUND)