Although these flags are harmless on linux, they
cause clang on darwin to issue an error about
unused flags (-lpthread). We only care about compilation
so we don't need them.
Change-Id: I0fc756e2f4d7a829d43b5aa912c4e4b24a802a1c
Reviewed-on: https://go-review.googlesource.com/21283
Reviewed-by: Robert Griesemer <gri@golang.org>