Crate to link and/or build SGX SDK

[nick-mobilecoin]

Currently the initial logic is looking for the SGX SDK at /opt/intel/sgxsdk and this logic is repeated in multiple places.
We should have a crate that handles this logic and optionally builds the sdk from source

Ideally there should be 2 options:

1. Linking to a pre-installed SGX SDK
2. Building the SGX SDK from source

One thought is to implement something like openssl-sys.
It has a build dependency on openssl-src.
When the vendored feature is enabled the openssl-src crate is built providing the OpenSSL libraries to link to at build time. The find_vendored.rs logic will be used to link the built OpenSSL library.
When vendored is not present then (find_normal.rs)[https://github.com/sfackler/rust-openssl/blob/master/openssl-sys/build/find_normal.rs] will be used to locate the system install of OpenSSL.