gcc 4.8 does not support C11 thread local storage. This is a bit
annoying, so add a hack to use the gcc specific __thread extension if
C11 TLS is not available.
(This is used for the extremely silly mpv-internal way hwdec modules
access some platform specific handles. Disabling it simply made
hwdec_vaegl.c always fail initialization.)
Fixes#2631.