crypto: explicitly enable auto cert chaining

OpenSSL enables this feature by default, but BoringSSL doesn't, because it's expensive and unnecessary (see boringssl/27 and boringssl/42). This change makes it so that when building node with BoringSSL, the behaviour matches OpenSSL's.

Electron will be building node.js with BoringSSL in future, and we've floated this patch on our fork (see electron/node#47).

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