add PREFIX option to configure to prefix of libraries.

The prefix controls the default output directory for headers, 
python modules and the prefix of libraries. By default, the 
prefix is set to ost.


git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@1929 5a81b35b-ba03-0410-adc8-b2c5c5119f08