Fix IPOPT check, hopefully

cmake/modules/FindIPOpt.cmake

@@ -26,9 +26,8 @@ find_package_handle_standard_args(hsl DEFAULT_MSG HSL_LIBRARY)
 find_package_handle_standard_args(dl DEFAULT_MSG DL_LIBRARY)
 find_package_handle_standard_args(Ipopt DEFAULT_MSG IPOPT_INCLUDE_DIR IPOPT_LIBRARY)
 
-set(HAVE_IPOPT ENABLE_IPOPT)
-
 if(IPOPT_FOUND)
+    set(HAVE_IPOPT ENABLE_IPOPT)
     set(DUNE_IPOPT_CFLAGS "-I${IPOPT_INCLUDE_DIR} -DENABLE_IPOPT=1")
     set_property(GLOBAL APPEND PROPERTY ALL_PKG_FLAGS "-I${IPOPT_INCLUDE_DIR}")
     set(IPOPT_LIBRARY ${IPOPT_LIBRARY})