The struct member function inside BOOST_MPL_ASSERT_MSG_IMPL was causing
multiple definitions of the same symbol when used inside e.g. template
functions.
An example of where this is a big issue is in the boost/geometry library
where you currently only can call each function once for each geometry
or you get hit by the multiple definitions linking error.
This fix* works around the issue by gracefully degrading to
BOOST_STATIC_ASSERT_MSG instead of failing during linking.
Robin Lindén
e44b2ae761