Configurable build variables

When compiling this crate there's no way to specify any build configuration for OpenSSL. Being able to specify the build configuration allows building a more hardened version of OpenSSL when some features are not required. This means both no-* configuration flags (for example, no-rc2), as well as -D configuration flags (for example -DOPENSSL_NO_HEARTBEATS). This feature would also need to be made available from the openssl crate.