[trivial] Sync ax_pthread with upstream draft

build-aux/m4/ax_pthread.m4

@@ -82,7 +82,7 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 22
+#serial 23
 
 AU_ALIAS([ACX_PTHREAD], [AX_PTHREAD])
 AC_DEFUN([AX_PTHREAD], [
@@ -212,7 +212,7 @@ case $host_os in
         ax_pthread_check_macro="_REENTRANT"
         ;;
 
-	aix* | freebsd*)
+        aix*)
         ax_pthread_check_macro="_THREAD_SAFE"
         ;;