Auto merge of #5865 - alexcrichton:vendor-openssl, r=matklad
Auto merge of #5865 - alexcrichton:vendor-openssl, r=matklad

Add a feature to build a vendored OpenSSL

This will be enabled through rust-lang/rust to ensure that for dist builds we
can build a Cargo with a vendored OpenSSL

rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,vojtechkral/cargo,vojtechkral/cargo,tsurai/cargo,eminence/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,tsurai/cargo,tsurai/cargo,rust-lang/cargo,eminence/cargo,tshepang/cargo,tsurai/cargo,rustyhorde/cargo,rustyhorde/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,vojtechkral/cargo,tshepang/cargo