Build: Stop using the $(shell pwd)

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
[snip]
Makefile.am:13: warning: shell pwd: non-POSIX variable name
Makefile.am:13: (probably a GNU make extension)
```