Log Message:
harfbuzz: update to 1.8.1.

Overview of changes leading to 1.8.1
Tuesday, June 12, 2018
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- Fix hb-version.h file generation; last two releases went out with wrong ones.
- Add correctness bug in hb_set_t operations, introduced in 1.7.7.
- Remove HB_SUBSET_BUILTIN build option.  Not necessary.