[libx11] set environment variable DEBIAN_FRONTEND

This will remove the following issue when running ```apt-get```.

```
debconf: unable to initialize frontend: Dialog
debconf: (No usable dialog-like program is installed, so the dialog
based frontend cannot be used. at
/usr/share/perl5/Debconf/FrontEnd/Dialog.pm line 76.)
debconf: falling back to frontend: Readline
debconf: unable to initialize frontend: Readline
debconf: (This frontend requires a controlling tty.)
debconf: falling back to frontend: Teletype
```
Merge pull request #4 from berendt/set_debian_frontend

[libx11] set environment variable DEBIAN_FRONTEND