Add flag BOOST_VARIANT_USE_RELAXED_GET_BY_DEFAULT.

[ealmansi]

Fixes #3530

Echoing from the linked issue: I don't have personal experience so as to judge whether this solution is reasonable or somewhat of a hack.

[chriseth]

I would rather avoid removing what looks like safety features. Could you please check if changing the line from

FunctionalInstruction const& instruction = boost::get<FunctionalInstruction const&>(_expr);

to

FunctionalInstruction const& instruction = boost::get<FunctionalInstruction>(_expr);

fixes it?

[leonardoalt]

I also had this problem and confirm that applying that change (removing const &) to that and 3 other occurrences leads to compilation working again.

[chriseth]

Closing this in favour of #3536