[variant] unreferenced formal parameter warning from VC9

[Richard Webb]

(i think i might have mentioned this before, but it still happens with the current code, so...)

Using Variant on VC9 with warning level 4 can result in an unreferenced parameter warning:

boost/variant/variant.hpp(289) : warning C4100: 'operand' : unreferenced formal parameter

This can be silenced by extending the existing Borland workaround to apply to VC as well. For example, change the existing:

#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x0551))
        operand; // suppresses warnings

to

#if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x0551)) || BOOST_WORKAROUND(BOOST_MSVC, >= 1400)
        operand; // suppresses warnings


Thanks,
Richard Webb