Also set HAVE_CSTDDEF=1 when registering package flags

cmake/modules/FindIPOpt.cmake

@@ -39,7 +39,7 @@ if(IPOPT_FOUND)
     endif(DL_FOUND)
 
     # register all related flags
-    dune_register_package_flags(COMPILE_DEFINITIONS "ENABLE_IPOPT=1"
+    dune_register_package_flags(COMPILE_DEFINITIONS "ENABLE_IPOPT=1" "HAVE_CSTDDEF=1"
                                 LIBRARIES "${IPOPT_LIBRARY}"
                                 INCLUDE_DIRS "${IPOPT_INCLUDE_DIR}")
 endif(IPOPT_FOUND)