build: Don't use HAVE_GTK_DOC to determine building docs
Now we use ENABLE_GTK_DOC like other projects and have updated configure according to the latest recommendations from the gtk-doc documentation. Additionally, we only use ENABLE_GTK_DOC to disable building ontology documentation because we use that before detection of the other tools we need when building documentation (such as dia and graphviz). Fixes GB#659995.
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