[concept_check.hpp] remove unused variable warning in msvc

[hxpro]

simple enough fix

[hxpro]

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So I suppose it helps if I actually attatched the patch ...

Related to this patch, would it be a good idea to promote
the unused_variable helper to a more public place? I belive
UBLAS uses a similar mechanism but not this actual one
because its kind of hidden,

[biochimia]

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After reviewing the patch it looks all of valid, correct,
appropriate, trivial and harmless.

The ignore_unused_variable_warning workaround is already
used in the file to patch in a lot of places. The fact that
it isn't used in this one looks like an oversight.

Ok to commit?

[rgarcia]

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Not sure why this was assigned to me...

[John Maddock]

This appears to be a duplicate of another (now closed and out of date issue).

Shutting down, please reopen if you can verify the problem is still in 1.35.

John Maddock.