threadlib: Fix link errors on FreeBSD 5.2.1/i386

[Bruno Haible]

On FreeBSD 5.2.1/i386, a testdir shows many link errors for pthread_* functions.
Most of these functions exist in libthr.so; there is no libpthread in this
version of FreeBSD.


2023-10-15  Bruno Haible  <bruno@clisp.org>

        threadlib: Fix link errors on FreeBSD 5.2.1/i386.
        * m4/threadlib.m4 (gl_PTHREADLIB_BODY): When pthread_kill could not be
        found in -lpthread, try -lthr.

diff --git a/m4/threadlib.m4 b/m4/threadlib.m4
index 303382881a..855e563d88 100644
--- a/m4/threadlib.m4
+++ b/m4/threadlib.m4
@@ -1,4 +1,4 @@
-# threadlib.m4 serial 39
+# threadlib.m4 serial 40
 dnl Copyright (C) 2005-2023 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -269,6 +269,15 @@ AC_DEFUN([gl_PTHREADLIB_BODY]
                    [Define if the pthread_in_use() detection is hard.])
              esac
            fi
+          ],
+          [dnl This is needed on FreeBSD 5.2.1.
+           AC_CHECK_LIB([thr], [pthread_kill],
+             [if test $gl_pthread_in_glibc = yes; then
+                LIBPMULTITHREAD=
+              else
+                LIBPMULTITHREAD=-lthr
+              fi
+             ])
           ])
       elif test $gl_pthread_api != yes; then
         # Some library is needed. Try libpthread and libc_r.