Build: Stop using the deprecated form of AM_INIT_AUTOMAKE

This deters the following warning.
```
$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.0 (Maipo)

$ autoconf --version
autoconf (GNU Autoconf) 2.69
[snip]

$ ./autogen.sh
configure.ac:6: warning: AM_INIT_AUTOMAKE: two- and three-arguments forms are deprecated.  For more info, see:
configure.ac:6: http://www.gnu.org/software/automake/manual/automake.html#Modernize-AM_005fINIT_005fAUTOMAKE-invocation
```