configure fails when building with gcc 11.1

Building kea with gcc 11.1 fails with - i'm sure there's a simple config fix for those with deeper understanding than mine:

...
checking thread support... no
configure: WARNING: unsupported C++11 feature
configure: retrying by adding --std=c++1x to g++ --std=c++0x
checking --std=c++1x support... no
configure: error: thread (a C++11 feature) is not supported
...