Add check that BOOST_HAS_LONG_LONG is supported on Darwin

before enabling the macro.  Apple helpfully supplies this
macro (__DARWIN_NO_LONG_LONG) if long long type is supported.
This to handle case where compiling with -pedantic causes
compiler errors because it enforces ISO compliance
(c++98 in this case).

This will fix several tests in develop and master and will clean conversion tests completely on Mac.