LibreSSL link failure on macOS

[fpagliughi]

It seems that linking will fail if LibreSSL is requested, but OpenSSL is installed in a default location.

The build finds the LibreSSL headers, but then attempts to link to the OpenSSL libraries. This is not a unique problem to our Paho library, but rather appears to be problematic using LibreSSL. It appears to be LibreSSL's own cmake file that finds and reports the wrong library files for linking.

On my machine, I installed LibreSSL under my home directory at /Users/fmp/usr/local. The current develop branch reports:

Use LibreSSL at 
SSL_INCLUDE_DIR: /Users/fmp/usr/local/include /usr/local/lib/libssl.dylib /usr/local/lib/libcrypto.dylib

Note that the include files are found at the correct location, but the libraries found in the default location of /usr/local.

The curl project seems to recommend unlinking openssl:
https://github.com/curl/curl/pull/11436/files

like:

% brew unlink openssl

I don't think this is something we could/should try to manage from this project, but it probably needs to be tested and documented here somewhere.