Provide build hook to allow environment config to set OpenSSL path

There is not a way for users to specify to the client a path to use for the OpenSSL library when the Python client is built. For the most part this is not a problem, but in some instances the ability to provide a specified path for the OpenSSL library would be useful.

Change:
If the PYCBC_OPENSSL_DIR environment variable is set, the build system will set the DOPENSSL_ROOT_DIR variable to the path provided in the PYCBC_OPENSSL_DIR environment variable (this should also propagate to the libcouchbase build).