Thread-local storage in GCC is platform-specific, and some platforms that
are otherwise perfectly capable of running mpv may lack TLS support in GCC.
This change adds a test for GCC variant of TLS and relies on its result
instead of assumption.
Provided that LLVM's `__thread` support is similar to GCC, the test is
called "GCC/LLVM TLS".
Signed-off-by: wm4 <wm4@nowhere>
Dmitrij D. Czarkoff
ee2ba599e7