If no argument is given then `"$1"` is an empty string, so no need to bother.

This fixes the following error received on OpenBSD when running `install.sh`.

```
$ sh install.sh
Copying fonts...
cp: cannot stat '%': No such file or directory
All Powerline fonts installed to /home/foo/.local/share/fonts
```

What's happening is that OpenBSD's `xargs` fails without the `-n1` option (i.e. copy the files one-at-a-time).

Adding the `-n1` seems to be compatible with Linux (I tried it on a CentOS system); probably holds for Mac OS X too (but I don't have one to test on).

fc-cache doesn't exist on osx :)

Further testing shows that the find command fails for GNU find under Ubuntu 11.10 and EL6.

This script was only copying over the pcf.gz files on OSX 10.9 and 10.10 since the find command is a little different.