[gdm] build: fix the build with automake 1.13

[Matthias Clasen <matthiasc src gnome org>]

commit 69ec35089783a52c70e42bace52fd3aab4b9f768
Author: Nuno Araujo <nuno araujo russo79 com>
Date:   Fri Jan 18 22:21:03 2013 +0100

    build: fix the build with automake 1.13
    
    In Automake 1.13, the long-deprecated macro AM_CONFIG_HEADER (deprecated
    since 2002) has been removed in favour of AC_CONFIG_HEADERS.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=692113

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index d939145..5912134 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,7 +38,7 @@ AC_HEADER_STDC
 
 AC_SUBST(VERSION)
 
-AM_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADERS(config.h)
 AC_CONFIG_MACRO_DIR([m4])
 
 # Documentation