Allow installation by non-root

Attempting to force root ownership for the man pages doesn't help users with
root access, it just makes it impossible for non-root users to install it.

Use cases:

* Non-admin installing to home directory

```
      $ export MANPATH=$MANPATH:$HOME/share/man
      $ ./configure --prefix=$HOME
      $ make install
```

* Packager using DESTDIR

```
      $ ./configure
      $ make install DESTDIR=/tmp/stdman
      $ tar czf stdman.tar.gz /tmp/stdman/* # Or a real packaging tool...
```