Find CUPS and link to it explicitly

Fixes linking error:

```
...
[100%] Building CXX object gui/CMakeFiles/boomaga.dir/qrc_translatorsinfo.cxx.o
Linking CXX executable boomaga
/tmp/boomaga/gui/kernel/printer.cpp:134: error: undefined reference to 'cupsGetDests'
/tmp/boomaga/gui/kernel/printer.cpp:136: error: undefined reference to 'cupsGetDest'
/tmp/boomaga/gui/kernel/printer.cpp:145: error: undefined reference to 'cupsGetOption'
/tmp/boomaga/gui/kernel/printer.cpp:146: error: undefined reference to 'cupsGetOption'
/tmp/boomaga/gui/kernel/printer.cpp:151: error: undefined reference to 'cupsGetPPD'
/tmp/boomaga/gui/kernel/printer.cpp:152: error: undefined reference to 'ppdOpenFile'
/tmp/boomaga/gui/kernel/printer.cpp:155: error: undefined reference to 'ppdMarkDefaults'
/tmp/boomaga/gui/kernel/printer.cpp:157: error: undefined reference to 'ppdPageSize'
/tmp/boomaga/gui/kernel/printer.cpp:166: error: undefined reference to 'ppdClose'
/tmp/boomaga/gui/kernel/printer.cpp:174: error: undefined reference to 'cupsGetOption'
/tmp/boomaga/gui/kernel/printer.cpp:178: error: undefined reference to 'cupsGetOption'
/tmp/boomaga/gui/kernel/printer.cpp:190: error: undefined reference to 'cupsFreeDests'
collect2: error: ld returned 1 exit status
make[2]: *** [gui/boomaga] Error 1
make[1]: *** [gui/CMakeFiles/boomaga.dir/all] Error 2
make: *** [all] Error 2
```