Makefile: Use same build rule for all targets

This reduces code duplication by defining a single compilation command,
which means that simple copy and paste fixes like this aren't needed:

        commit 58c6e58d49b36fa6bbb6e2b99d5687356e4040df
        Author: Dhiru Kholia <dhiru@openwall.com>
        Date:   Fri Aug 23 05:06:22 2013 +0530

            fix trivial build problems