Use proxy information from Windows when downloading files

[ras0219-msft]

Currently, portfiles use CMake's built-in downloading function to acquire sources. This function is backed by curl, which doesn't use Windows' system-wide proxy settings and instead requires special environment variables (http_proxy, https_proxy, ftp_proxy, ...).

Inside vcpkg.exe, we should detect the Windows settings and fill in these environment variables before calling CMake so users get a much cleaner out-of-the-box experience.

[KindDragon]

Inside vcpkg.exe, we should detect the Windows settings and fill in these environment variables before calling CMake so users get a much cleaner out-of-the-box experience.

But in this case, the user will still have to specify a username and password?

[ras0219-msft]

Perhaps; curl will use the username/password information from the environment variable so we could prompt for credentials (or, more nicely, take it from a command line argument/config file).

[PhoebeHui]

I think microsoft/vcpkg-tool#49 has implemented this, @ras0219-msft, please reopen this issue if i'm wrong.