Fixup building on Ubuntu 18.04

[laerreal]

Build errors examples:

../src/utils/util.h:17:19: error: ‘isnan’ is not a member of ‘std’
         if (!std::isnan(v1) && !std::isnan(v2)) return qMin<double>(v1, v2);
                   ^~~~~

../src/tables/heightmaptablemodel.cpp:20:24: error: ‘NAN’ was not declared in this scope
             row.append(NAN);
                        ^~~