Explicit specification of attribute value type is necessary when it cannot
be inferred from the function object type. With C++20 removing argument type
typedefs from standard function objects, like std::less, and compilers following
suit (e.g. future MSVC versions), it is better to update the example so that
it is compatible with future compilers.
Related to https://github.com/boostorg/log/issues/105.
Some tests pull in pthread rwlock and therefore need _XOPEN_SOURCE=600
or higher to compile. Added platform defines to tests and examples
for good measure.
For MinGW worked around Boost.Build error when BOOST_LOG_WITHOUT_EVENT_LOG
is defined.
