ixion @0.17.0: fatal error: 'variant' file not found

[ryandesign (Ryan Carsten Schmidt)]

​https://build.macports.org/builders/ports-10.13_x86_64-builder/builds/134192/steps/install-port/logs/stdio

  CXX      address.lo
  CXX      address_iterator.lo
  CXX      calc_status.lo
  CXX      cell.lo
  CXX      cell_access.lo
  CXX      compute_engine.lo
  CXX      concrete_formula_tokens.lo
  CXX      config.lo
  CXX      debug.lo
  CXX      dirty_cell_tracker.lo
In file included from cell.cpp:16:
../../include/ixion/matrix.hpp:16:10: fatal error: 'variant' file not found
#include <variant>
         ^~~~~~~~~
In file included from cell_access.cpp:10:
../../include/ixion/model_context.hpp:16:10: fatal error: 'variant' file not found
#include <variant>
         ^~~~~~~~~
1 error generated.
make[3]: *** [cell.lo] Error 1
make[3]: *** Waiting for unfinished jobs....

Perhaps it requires a particular recent C++ standard which could be indicated with compiler.cxx_standard.

Also, silent rules should be disabled.

[Tatsh (Andrew Udvare)]

​https://github.com/macports/macports-ports/pull/13465

[Tatsh (Andrew Udvare)]

In b74dbed4e80e5762b284ab46030d58f479d646cb/macports-ports (master):

ixion: set compiler.cxx_standard

Possibly fixes #64278