cmake CMP0077 warning

This is showing up with cmake 3.18.4:

CMake Warning (dev) at suite/tests/CMakeLists.txt:55 (option):
  Policy CMP0077 is not set: option() honors normal variables.  Run "cmake
  --help-policy CMP0077" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  For compatibility with older versions of CMake, option is clearing the
  normal variable 'TEST_LONG'.
This warning is for project developers.  Use -Wno-dev to suppress it.