Fix USE_OPENSSL is ignored

[temideoye]

Currently, -DUSE_OPENSSL has no effect. In its stead, an undocumented variable – ENABLE_OPENSSL_ENCRYPTION – is what gets evaluated to verify if OpenSSL was requested. ENABLE_OPENSSL_ENCRYPTION defaults to undefined/OFF, which invariably means OpenSSL is actually never used, regardless of the value of USE_OPENSSL. This push proposes a fix by linking USE_OPENSSL to ENABLE_OPENSSL_ENCRYPTION.

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[jmklix]

https://github.com/aws/aws-sdk-cpp/issues/1888