add minimum visibility support

[jhunold]

Just to get the tests going again

[apolukhin]

This is related to the changes in boostorg/boost#190
Note that you also have to update all the exceptions in boost/graph/exception.hpp https://github.com/boostorg/graph/blob/4c752ee2f11c249e0d5b1fab8f0ef9da3428f575/include/boost/graph/exception.hpp#L18 by adding BOOST_SYMBOL_VISIBLE

See https://www.boost.org/doc/libs/1_65_1/libs/config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.macros_for_libraries_with_separate_source_code for more info

[jhunold]

Well, this is just to make the unit test compile again (first shot). And yes, I know the relevant macros because I added them a long time ago.

[jeremy-murphy]

So, ummm... is this still relevant, or has it been superseded by other changes?