client: Added HAS_OPENSSL preprocessor definition. If not set, SSL-related...

client: Added HAS_OPENSSL preprocessor definition. If not set, SSL-related code won't be included and XtreemFS does not depend on OpenSSL.

By default, CMake will always set the definition for OpenSSL. It has to be manually commented in the CMake configuration cpp/CMakeLists.txt to compile XtreemFS without OpenSSL.