Use Platform Root CA by Default

PR is ready for review: https://github.com/couchbaselabs/couchbase-cxx-client/pull/281

check OPENSSLDIR value

For example OpenSSL module inside Ruby interpreter does it like this: https://github.com/ruby/ruby/blob/cc8ff8b50d445b0621fef9f3dce0da02ec3d406f/ext/openssl/ossl_x509store.c#L395-L418

Python also has similar logic (maybe even something windows-specific, but I haven't checked):
https://github.com/python/cpython/blob/00464bbed66e5f64bdad7f930b315a88d5afccae/Lib/ssl.py#L380-L392

https://www.openssl.org/docs/manmaster/man3/X509_STORE_set_default_paths.html

X509_STORE_set_default_paths_ex() is somewhat misnamed, in that it does not set what default paths should be used for loading certificates. Instead, it loads certificates into the X509_STORE from the hardcoded default paths. The library context libctx and property query propq are used when fetching algorithms from providers.

OpenSSL initialisation code lives here: https://github.com/couchbaselabs/couchbase-cxx-client/blob/78e3e889f01ee3f082a05d031d9f9ec7c2004195/core/cluster.hxx#L344-L398

The task is to find out what we can do regarding the subject and update this place.