isnanl: Avoid autoconf warning

[Bruno Haible]

When creating a testdir with the modules 'modfl' and 'log10l', I see this
warning:

executing autoconf
configure.ac:342: warning: AC_REQUIRE: 'gl_FUNC_ISNANL' was expanded before it 
was required
configure.ac:342: 
https://www.gnu.org/software/autoconf/manual/autoconf.html#Expanded-Before-Required
glm4/logl.m4:7: gl_FUNC_LOGL is expanded from...
glm4/log10l.m4:7: gl_FUNC_LOG10L is expanded from...
configure.ac:30: gl_INIT is expanded from...
configure.ac:342: the top level

This patch fixes it.


2024-01-18  Bruno Haible  <bruno@clisp.org>

        isnanl: Avoid autoconf warning.
        * m4/isnanl.m4 (gl_FUNC_ISNANL): Declare through AC_DEFUN_ONCE.

diff --git a/m4/isnanl.m4 b/m4/isnanl.m4
index b04d9bc984..9f7a5f133c 100644
--- a/m4/isnanl.m4
+++ b/m4/isnanl.m4
@@ -1,10 +1,10 @@
-# isnanl.m4 serial 25
+# isnanl.m4 serial 26
 dnl Copyright (C) 2007-2024 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
 
-AC_DEFUN([gl_FUNC_ISNANL],
+AC_DEFUN_ONCE([gl_FUNC_ISNANL],
 [
   AC_REQUIRE([gl_MATH_H_DEFAULTS])
   ISNANL_LIBM=