gromacs/gromacs-plumed: build requires macOS 10.13+; possible candidate for macports-libcxx

[mascguy (Christopher Nielsen)]

These ports fail to build on macOS releases earlier than 10.13, with the following errors:

src/gromacs/modularsimulator/computeglobalselement.cpp:393:62:
  error: 'value' is unavailable: introduced in macOS 10.13
    return std::any_cast<ISimulatorElement*>(cachedValue.value());
                                                         ^
/opt/local/libexec/llvm-9.0/bin/../include/c++/v1/optional:944:33:
  note: 'value' has been explicitly marked unavailable here
    constexpr value_type const& value() const&
                                ^

src/gromacs/modularsimulator/computeglobalselement.cpp:393:21:
  error: 'any_cast<gmx::ISimulatorElement *>' is unavailable: introduced in macOS 10.13
    return std::any_cast<ISimulatorElement*>(cachedValue.value());
                ^
/opt/local/libexec/llvm-9.0/bin/../include/c++/v1/any:581:12:
  note: 'any_cast<gmx::ISimulatorElement *>' has been explicitly marked unavailable here
    _ValueType any_cast(any const & __v)
               ^

However, the use of macports-libcxx might be on option on 10.12 and earlier, to fix these issues. @kencu would be able to say for sure, though.

In the interim, we should set known_fail as appropriate, to save wasted buildbot time.

[mascguy (Christopher Nielsen)]

I'll take this, as I'm working on fixing support for macports-libcxx within PG legacysupport.

[Christopher Nielsen <mascguy@…>]

In 7a40798dec5597b30008e07601ffefbf60a01fcb/macports-ports (master):

gromacs/gromacs-plumed: use macports-libcxx, for 10.12 and earlier
Fixes: #63818
See: #63869