AppleClang does not support the cxx_thread_local feature, even
though it is based on a Clang version which does support the
feature.

 http://stackoverflow.com/a/23850891/2428389

A possible reason for that is that thread_local might be used as
a variable in existing Apple SDK headers.

Extend the WriteCompilerDetectionHeader module to generate a define
for that feature with portability fallbacks.  For the avoidance of
making it easy to write code which looks correct but which has odd
runtime behavior, don't set the define symbol at all if no
equivalent keyword is known.