GCC-3.0.4 (pre) on AIX requires pthreads

[David Edelsohn]

	Grepping for "thread model" in a GCC configure on AIX produces:

checking for thread model used by GCC... single
checking for thread model used by GCC... posix
checking for thread model used by GCC... single
checking for thread model used by GCC... single
checking for thread model used by GCC... single
checking for thread model used by GCC... posix
checking for thread model used by GCC... posix
checking for thread model used by GCC... posix

#define THREAD_MODEL_SPEC "%{pthread:posix}%{!pthread:single}"

David