Syntax error in SWIG using __thread keyword

Question

I am trying to make some code thread safe for use with pthreads. This code is written in C++, but is linked to using SWIG. g++ compiles this correctly, but when swig tries to create a wrapper, I get:

fast_alloc.hh:109: Error: Syntax error in input(3)

The original, unsafe code that compiles correctly is:

static void *freeLists[Num_Buckets];

and the error occurs when I change it to:

static __thread void *freeLists[Num_Buckets];