Configure OpenSSL data dir on vendored builds.

[DanielSidhion]

Currently, any build using Nix fails if it's attempting to build a static binary (musl target) that uses this crate with the vendored flag. This happened because the openssl-src crate looked for the openssl dir using a hardcoded path, and was recently fixed in alexcrichton/openssl-src-rs#220.

This PR bumps the version of openssl-src to get the fix, and propagates the value of the OPENSSL_DIR environment variable to openssl-src if it's set. This is all that's needed to unblock static musl builds on Nix.

[sfackler]

The OPENSSL_DIR environment variable is already used for an entirely different purpose in openssl-sys.

[DanielSidhion]

My apologies for the initial changes in this PR - I just dug more into this matter and understand better why not use OPENSSL_DIR, and also what was the actual build failure I was having. The build failure turned out to be a missing perl during the build with Nix (the error message made it look like a completely different issue).

As for not using OPENSSL_DIR - I'm leaning towards creating and documenting a new environment variable for this. This variable would only be used in a vendored build to configure the directory that OpenSSL uses by default to look for configuration file and certificates. This crate's current documentation makes it seem like OpenSSL will not look in any place on a vendored build, but it does. This is especially important on vendored builds with Nix, since /usr/local/ssl (the default directory as configured by openssl-src) may contain files for a completely different version of OpenSSL, and/or not be controlled by whoever runs the binary with the vendored build.

[Skepfyr]

This seems neat, is clearly needed, and is documented clearly, @sfackler shout of you disagree but I'm hitting merge. Thanks for this!