mingw32-curl: add mgwport file

Don't use configure. The reason is that I found no way to enable
IPv6 and WINSSL with it.

$ curl --version
curl 7.28.1 (i386-pc-win32) libcurl/7.28.1 WinSSL zlib/1.2.7
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp smtp smtps telnet tftp
Features: AsynchDNS GSS-Negotiate IPv6 Largefile NTLM SPNEGO SSL SSPI libz

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

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