[bugfix] Make sure to use tolower from c

Matrixmarket so far uses std::transform(..,..,.., tolower). However it only works with the C function tolower, not with std::tolower (which is a template). The patch explicitly chooses the C version. (Of course, this only breaks if somebody draws std::tolower or similar into the scope, which I would generally discourage. But the resulting failure can be avoided, so we should avoid it.)