OSF supports pthreads, per Philip Yarra.

tglsfdc · tglsfdc · commit bbac4188f9ea · 2003-07-15T17:28:36.000Z
diff --git a/src/template/osf b/src/template/osf
@@ -4,3 +4,6 @@ else
   CC="$CC -std"
   CFLAGS='-O4 -Olimit 2000'
 fi
+SUPPORTS_THREADS=yes
+THREAD_CFLAGS="-pthread"
+
