mingw32-curl: Switch to OpenSSL and install ca-bundle.crt

The winssl version has trouble when trying to clone from an internal
network machine when the proxy access is unavailable due to checking for
certificate revocation.

This now is in line with msysgit, see

    https://github.com/msysgit/msysgit/commit/3a9151cddd239aab12436ec42b042720c1340f33

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>