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.
Adding calc_data to BUILD_REQUIRES (when we build calc for a
secondary arch) is required to avoid a build failure when an
outdated version of calc_data is already installed.
waddlesplash