Renovate autoconf for compatibility with 2.71

see https://github.com/varnishcache/varnish-cache/pull/3804 for context

The C compiler checks have been removed as they are already
contained in VARNISH_PREREQ

configure.ac

-AC_PREREQ(2.68)
+AC_PREREQ([2.71])
 AC_COPYRIGHT([Copyright (c) 2019 UPLEX - Nils Goroll Systemoptimierung])
-AC_INIT([libvdfp-pipe], [trunk], [varnish-support@uplex.de], [vdfp-pipe])
+AC_INIT([libvdfp-pipe],[trunk],[varnish-support@uplex.de],[vdfp-pipe])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_SRCDIR(src/vdfp_pipe.vcc)
-AM_CONFIG_HEADER(config.h)
-AC_CANONICAL_SYSTEM
+AC_CONFIG_HEADERS([config.h])
+AC_CANONICAL_TARGET
 AC_LANG(C)
 
 AM_INIT_AUTOMAKE([1.12 -Wall -Werror foreign parallel-tests])
@@ -14,15 +14,6 @@ AM_PROG_AR
 LT_PREREQ([2.2.6])
 LT_INIT([dlopen disable-static])
 
-AC_PROG_CC
-AC_PROG_CC_STDC
-if test "x$ac_cv_prog_cc_c99" = xno; then
-	AC_MSG_ERROR([Could not find a C99 compatible compiler])
-fi
-AC_PROG_CPP
-AC_PROG_CXX
-AC_PROG_RANLIB
-
 # To get absolute paths in the vtc tests.
 AC_PATH_PROG([CAT], [cat], [])
 AC_PATH_PROG([TR], [tr], [])