Start modernizing configure

* configure.ac: reanmed configura.in, use AC_CONFIG_HEADERS instead of
  AM_CONFIG_HEADER

configure.in → configure.ac

@@ -3,7 +3,7 @@ dnl Process this file with autoconf to produce a configure script.
 AC_INIT([libvirt], [0.7.5])
 AC_CONFIG_SRCDIR([src/libvirt.c])
 AC_CONFIG_AUX_DIR([build-aux])
-AM_CONFIG_HEADER([config.h])
+AC_CONFIG_HEADERS([config.h])
 dnl Make automake keep quiet about wildcards & other GNUmake-isms
 AM_INIT_AUTOMAKE([-Wno-portability])