Details:
- Disabled the _self() and _equal() extensions to the bli_pthread API
introduced in d479654. These functions were disabled after I realized
that they aren't actually needed yet. Thanks to Devin Matthews for
helping me reason through the appropriate consumer code that will
appear in BLIS (eventually) in a future commit. (Also, I could never
get the Windows branch to link properly in clang builds in AppVeyor.
See the comment I left in the code, and #485, for more info.)