Fix build failure caused by ‘Generate upcase and downcase tables from Unicode’
The [5ec3a584: Generate upcase and downcase tables from Unicode data] commit broke bootstrap from a truly clean tree (e.g. a fresh clone or one created with ‘make extraclean’), see <http://hydra.nixos.org/build/48774928>. The failure was caused by characters.el trying to read Unicode property tables which aren’t available so early in the build process. Wrap the part that requires Unicode property tables in a condition checking if those are available. If they aren’t they case and syntax tables won’t be fully set but later on, the characters.el file will be evaluated again and this time with Unicode properties available so final Emacs ends up with the exact same case and syntax tables.
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