Description

Omit the 'curl' library if DD_TRACE_TRANSPORT is set to "none"

Motivation

There is clearly the intent to be able to not include a default transport, by setting DD_TRACE_TRANSPORT=none.
However, two things were missing:

1. To ignore the bundled curl library from the build
2. to replace the default_http_client_curl.cpp with default_http_client_null.cpp

Integrations that provide their own transport may not want to build and link a whole unused library, along with any runtime library dependencies it makes.

Additional Notes