Make it more obvious that XPDF headers have unstable API/ABI
Yesterday again a distribution packager complained that we're changing the internal API and i had to tell him that ENABLE_XPDF_HEADERS meant "Install unsupported xpdf headers.".
By renaming the option to ENABLE_UNSTABLE_API_ABI_HEADERS maybe i get to live a bit more happily by people not coming and asking why we changed API/ABI or something we expecitely tell we will do.
Comments?