That's really a C related comment and all it doing is subbing in implementation specific pre-standard versions for the benefit of compatibility with compilers prior to the introduction of the feature in the C11 standard.
The standard calls it "_Thread_local", pre-standard MSVC/icc(win)/borland called it __declspec(thread), and pre-standard GCC/icc(linux)/clang/sun called it __thread.
Code would also be free to just use _Thread_local and say that a C11 conforming compiler is required.
