Add support for "make DESTDIR=..." for packagers

This commit adds support for using `DESTDIR`, which is used by software packagers when building RPMs and other packages. When `DESTDIR` is set, it is used as a prefix for the install path, but when unset, the software installs to the system path (in this case, `/usr/local/bin`).

Example usage taken from an RPM spec file:

```
make install DESTDIR=%{buildroot}
```

This is important when packaging because you need to install the output to a contained area in order to build a clean package from it, rather than install onto the system and then attempt to pick the files back out to package them.