configure: Make MSVC version grabbing more robust

When running plain "cl", to get the MSVC version, it prints the
version header on stderr, while the usage instructions are printed
on stdout. Usually, the version on stderr gets flushed first,
so "head -n1" gets the line it expects, but some times (in particular
when running MSVC wrapped in wine), it can get the usage line
first.

Redirect stdout to /dev/null, so we only grab the version among
the lines printed to stderr. This should make the version number
grabbing more robust.

At least all relevant versions of MSVC seem to print this specifically
to stderr, not stdout (so we don't risk to miss it); checked down
to MSVC 2010.

Signed-off-by: Martin Storsjö <martin@martin.st>

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