Expose DEP_ var to dependents with path to OpenSSL headers

[alex]

Use case is here: pyca/cryptography#6695 (comment)

If there was a DEP_OPENSSL_HEADERS (or something), we could ensure that every part of the dependency tree is correctly compiling against the same copy of OpenSSL.

[sfackler]

This should exist as DEP_OPENSSL_INCLUDE (https://github.com/sfackler/rust-openssl/blob/master/openssl-sys/build/main.rs#L84), but I think that may not be set properly if the OpenSSL is discovered via homebrew or pkg-config. #1573 includes a refactor to that logic that should make a fix here easy after it lands.

[alex]

Revised request, document that :-)