Use vsnprintf instead of the potentially dangerous sprintf to avoid

Use vsnprintf instead of the potentially dangerous sprintf to avoid
warnings. Using snprintf directly would be a little bit more tricky,
because once again Microsoft decided to avoid followind standards and
provide _snprintf instead. We could use this, too, but this would
require an additional autoconf check, which I'd like to avoid, if
possible.

Note: If VS *still* issues warnings, but this time about vsnprintf,
somebody should add some pragmas or whatever is needed to shut up that
warning, it would be silly.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@783 7f0cb862-5218-0410-a997-914c9d46530a