For atomic shared_ptr, we only need to know if it compiles.
NFC, but I think CMAKE_REQ_LIBS is supposed to be set to something derived from FindThreads...
Document what TLS tests returning 1 mean.

Compiler and OS seem to run out-of-sync with eachother, with the compiler
happily accepting and generating TLS variables, but the dynamic linker
not correctly resolving them.
So this test needs to actually run without error, for a functioning TLS
implementation.