Partially revert "Replace static with thread_local when used for caching"

Partially revert "Replace static with thread_local when used for caching"

As noticed by Stephan, clang on Mac is nowhere near to support thread_local for
some reason:
http://stackoverflow.com/questions/28094794/why-does-apple-clang-disallow-c11-thread-local-when-official-clang-supports

It does not look realistic to me to provide a configure-based alternative. The
solution is to not use thread_local until it is reasonably supported on
Mac. According to sources, this means requiring Xcode >= 8.

This reverts commit 82d4f1a446260333ef1590f3622196e7c76e414d partially.