compilation error on Ubuntu 18.04 with ROS Melodic

[chataign]

/.../catkin_ws/src/laser_drivers/sicktoolbox2/include/sicktoolbox2/SickLD.hh:72:25: error: ‘constexpr’ needed for in-class initialization of static data member ‘const double SickToolbox::SickLD::SICK_MAX_SCAN_ANGULAR_RESOLUTION’ of non-integral type [-fpermissive]
static const double SICK_MAX_SCAN_ANGULAR_RESOLUTION = 0.125; ///< Minimum valid separation between laser pulses in active scan ares (deg)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/.../catkin_ws/src/laser_drivers/sicktoolbox2/include/sicktoolbox2/SickLD.hh:73:25: error: ‘constexpr’ needed for in-class initialization of static data member ‘const double SickToolbox::SickLD::SICK_DEGREES_PER_MOTOR_STEP’ of non-integral type [-fpermissive]
static const double SICK_DEGREES_PER_MOTOR_STEP = 0.0625; ///< Each odometer tick is equivalent to rotating the scan head this many degrees

static constexpr double works