Replace ancient m4/acx_pthread.m4 with m4/ax_pthread.m4

configure.ac

@@ -149,9 +149,8 @@ AS_IF([test "$with_protoc" != "no"], [
 ])
 AM_CONDITIONAL([USE_EXTERNAL_PROTOC], [test "$with_protoc" != "no"])
 
-ACX_PTHREAD
-AM_CONDITIONAL([HAVE_PTHREAD], [test "x$acx_pthread_ok" = "xyes"])
-
+AX_PTHREAD
+AM_CONDITIONAL([HAVE_PTHREAD], [test "x$ax_pthread_ok" = "xyes"])
 # We still keep this for improving pbconfig.h for unsupported platforms.
 AC_CXX_STL_HASH