gst-doc.m4 has the following test for gtk-doc:

    if perl <<EOF ; then
      exit (("$gtk_doc_version" =~ /^[[0-9]]+\.[[0-9]]+$/) &&
            ("$gtk_doc_version" >= "$gtk_doc_min_version") ? 0 : 1);
EOF

Currently, gtk-doc is version 0.10, and $gtk_doc_min_version is 0.7.  The
perl test "0.10" > "0.7" will return FALSE, as it does a lexical
comparison.  My perl is too rusty to propose a fix.