Add autogen.sh

Boilerplate/template autogen.sh. This is useful since it sets
prefix to /usr, which will help test changes easily for devs.

autogen.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+set -e
+
+autoreconf --force --install --symlink --warnings=all
+
+args="\
+--sysconfdir=/etc \
+--localstatedir=/var \
+--prefix=/usr \
+--enable-silent-rules"
+
+./configure CFLAGS="-g -O2 $CFLAGS" $args "$@"
+make clean