Fix build failure caused by ‘Generate upcase and downcase tables from Unicode’

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.