x86_64 is used as a baseline: the "x86_64" entry, whatever status it has,
is transformed into "all", and then the other entries in ARCHITECTURES
either dropped or rearranged appropriately.
* Drop lib:libcppunit from REQUIRES as it is not needed.
* lib:librevenge_{stream,generators} do not seem to be needed, but
we can keep them, since lib:librevenge already pulls librevenge.
* Add libxml2 because libepubgen now needs it.
* Sort {BUILD_,}REQUIRES.
* Switch SOURCE_URI to tar.xz.
waddlesplash