Replace obsolete automake AM_CONFIG_HEADER.

This fixes the following complaint by autoconf 2.69-1, automake 1.13.1-1.

:  configure.ac:80: error: 'AM_CONFIG_HEADER': this macro is obsolete.
:      You should use the 'AC_CONFIG_HEADERS' macro instead.
:      /usr/share/aclocal-1.13/obsolete-err.m4:12: AM_CONFIG_HEADER is expan
:      configure.ac:80: the top level

Automake 1:1.11.3-1ubuntu2, autoconf 2.68-1ubuntu2 don't even emit a warning
without, and work just fine with this patch.

Signed-off-by: Christian Vogel <vogelchr@vogel.cx>
diff --git a/configure.ac b/configure.ac
index 24ddd0c..fbc83f3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -77,7 +77,7 @@
 CHECK_TM_INCLUDES_TM_GMTOFF
 
 dnl Generate the output
-AM_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADER(config.h)
 
 AC_ARG_ENABLE(talloc,
 	[AS_HELP_STRING(