mingw32-curl: add mgwport file

The build method does not use configure. The reason is
that I found no way to enable IPv6 and WINSSL with it.

SSL certificate validation is done using WINSSL and the
Windows Certificate Store and not with a shipped cacert.pem.

Build flags inspired by https://github.com/msysgit/msysgit/commit/37e42ab860ae555d2ba4b0e3e014a0bc6860f0d5

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>