Use presence of libxml / curl config programs as test for presence

Symptom was that after configuring, curl was present but not used for
WWW fetching.

Original commit that changed this was

    commit 7da03ba5cd6e45ea41afebd4955acf6e96e9d622
    Date:   Fri Apr 5 19:01:55 2013 +0100

which was Raptor 2.0.10 onwards