[PATCH] Tweak to NetBSD -pthread option

[Loren James Rittle]

> Fixes an oversight when I initially checked this in (namely, -pthread
> should only be present when threads support is actually present in
> the compiler).

This is a good fix that more ports should properly support to aid
users of the compiler.

> Committed to mainline.
>
>	* config.gcc (*-*-netbsd*): Add NETBSD_ENABLE_PTHREADS to
>	tm_defines if pthreads are enabled.
>	* config/netbsd.h (LIB_SPEC): Only support the -pthread option
>	if NETBSD_ENABLE_PTHREADS is defined.

FYI, when FreeBSD made a similar change, we were asked to only place
new defines for a port in a new file added to tm_file.

Regards,
Loren