netgen @6.2.2004: fatal error: 'optional' file not found

[ryandesign (Ryan Carsten Schmidt)]

​https://build.macports.org/builders/ports-10.12_x86_64-builder/builds/120640/steps/install-port/logs/stdio

/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_math_netgen/netgen/work/netgen-6.2.2004/libsrc/include/mystdlib.h:22:10: fatal error: 'optional' file not found
#include <optional>
         ^~~~~~~~~~

Optional is a C++ 17 feature so this port should use compiler.cxx_standard 2017.

I don't see any -std=c++17 flags in the build so the port shoudl also use configure.cxxflags-append -std=c++17`.

Note that ports that use "optional" will fail to build on 10.13 until ​https://github.com/macports/macports-base/pull/179 is merged, a new version of base is released, and the clang-9.0 port receives a revbump (see ​https://github.com/macports/macports-ports/pull/6971).

Note that ports that use "optional" will never build in 10.7 thru 10.12 inclusive because their libc++ doesn't support it.