windows: fix build with mingw64

tested ok with curl-7.38.0 openssl 1.0.1j

was really not easy to setup, ssl config:

 CROSS_COMPILE="x86_64-w64-mingw32-" ./Configure mingw64 no-asm no-shared
 make && make install

curl config:

 extraopts=--enable-ipv6
 extraflags="-DOPENSSL_NO_ASM -D_THREAD_SAFE"
 openssl=/usr/local/ssl
 CROSS_COMPILE="x86_64-w64-mingw32-" ./configure --enable-shared=no \
 --disable-manual --without-libssh2 --disable-rtsp --disable-ldap \
 --disable-dict --disable-pop3 --disable-ftp --disable-telnet --disable-tftp \
 --disable-smtp --disable-imap --disable-ldaps --disable-gopher --with-zlib \
 --with-ssl=$openssl --with-libssl-prefix=$openssl CPPFLAGS="$extraflags" ${extraopts}

Signed-off-by: Tanguy Pruvot