tests: propagate CPPFLAGS into build invocations

With openssl installed by brew on macOS, OpenSSL headers are not in a
normal place.  I can fiddle with LDFLAGS/CPPFLAGS to get them available,
but then the `./configure` step succeeds and build fails.

Propagating the CPPFLAGS into the generated Makefile lets the build
succeed and we get a `client-ssl` binary output.