Fix crash in head when launching GUI after building without cairo

Upstream made pango and cairo hard dependencies since the last stable release.

Here is the crash:

```
$ /usr/local/bin/fontforge 
Copyright (c) 2000-2012 by George Williams. See AUTHORS for contributors.
 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
 with many parts BSD <http://fontforge.org/license.html>. Please read LICENSE.
 Executable based on sources from 18:28 CEST 29-Aug-2013-ML-D.
 Library based on sources from 18:28 CEST 29-Aug-2013.
 Based on source from git with hash:
PythonUI_Init()
CreateAllPyModules()
fontforge(83479,0x7fff74609180) malloc: *** error for object 0x7fc1acd01ce0: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap: 6
```