In the log, I see:
:info:configure Error when bootstrapping CMake:
:info:configure Cannot find appropriate C++ compiler on this system.
:info:configure Please specify one using environment variable CXX.
:info:configure See cmake_bootstrap.log for compilers attempted.
:info:configure ---------------------------------------------
:info:configure Log of errors: /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_devel_cmake-bootstrap/cmake-bootstrap/work/cmake-3.9.6/Bootstrap.cmk/cmake_bootstrap.log
Although we would need to see the contents of cmake_bootstrap.log to know for sure, this is probably #70750, given that you also just filed #71065 where that was definitely the cause. See ProblemHotlist#clts16 for the solution.