threadlib: Avoid autoconf warning "was expanded before it was required"

[Bruno Haible]

Creating a testdir for the module 'tls', I got an autoconf warning that
gl_THREADLIB was expanded before it was required.

This patch fixes it.


2019-06-25  Bruno Haible  <br...@clisp.org>

        threadlib: Avoid autoconf warning "was expanded before it was required".
        * modules/threadlib (configure.ac): Require gl_THREADLIB.

diff --git a/modules/threadlib b/modules/threadlib
index 740f62f..0a582c2 100644
--- a/modules/threadlib
+++ b/modules/threadlib
@@ -13,7 +13,7 @@ configure.ac-early:
 gl_THREADLIB_EARLY
 
 configure.ac:
-gl_THREADLIB
+AC_REQUIRE([gl_THREADLIB])
 
 Makefile.am:
 lib_SOURCES += glthread/threadlib.c