Soft retirement of autogen.sh

We shouldn't need to care about all the details when autoreconf(1) can
do that for us. The autoworld has changed over the last decade and this
only affects Varnish developers since we ship release archives with a
turnkey configure script. So let's see how much wreckage this change will
cause.

The script remains until its refcount drops to zero.

In order to get early feedback for MacOS the relevant Travis CI job is
already updated.

.travis.yml

@@ -86,7 +86,7 @@ jobs:
           update: true
       before_script:
         - export PATH="/usr/local/opt/sphinx-doc/bin:$PATH"
-        - ./autogen.sh
+        - autoreconf -i
         - ./configure
       script: *script-common
     - <<: *test-linux

autogen.des

@@ -23,7 +23,7 @@ else
 fi
 
 rm -f configure
-(cd $SRCDIR && . ./autogen.sh 2>&1 | egrep -v "(subdir-objects|is in a subdirectory)")
+(cd $SRCDIR && autoreconf -i 2>&1 | egrep -v "(subdir-objects|is in a subdirectory)")
 
 # autoconf prior to 2.62 has issues with zsh 4.2 and newer
 CONFIG_SHELL=/bin/sh