[telegnome] Remove some autoconf deprecation warnings

[Colin Watson <cjwatson src gnome org>]

commit a5e7e82e58e66a6bf9a44878c22aff9cc5f8029c
Author: Colin Watson <cjwatson debian org>
Date:   Mon Aug 30 00:10:25 2021 +0100

    Remove some autoconf deprecation warnings
    
    * configure.ac: Replace AM_CONFIG_HEADER with AC_CONFIG_HEADERS.  Remove
    ancient and unused AC_ISC_POSIX and AC_HEADER_STDC.

 configure.ac | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 2e3597a..bc84b34 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3,7 +3,7 @@ AC_PREREQ(2.50)
 AC_CONFIG_SRCDIR([src/main.vala])
 AC_CONFIG_MACRO_DIRS([m4])
 
-AM_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADERS([config.h])
 AM_INIT_AUTOMAKE([1.13 dist-xz no-dist-gzip])
 
 AM_MAINTAINER_MODE
@@ -17,10 +17,8 @@ AX_COMPILER_FLAGS
 YELP_HELP_INIT
 GLIB_GSETTINGS
 
-AC_ISC_POSIX
 AC_PROG_CC
 AC_PROG_CPP
-AC_HEADER_STDC
 AC_PROG_INSTALL
 
 AC_DEFINE_UNQUOTED([SYSCONFDIR], "$sysconfdir", [System configuration directory.])