turn on fvisibility=hidden on Mac

fvisibility=hidden decreases startup time of the program
considerably. Please try if it is possible to turn it on
by default on other platforms.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@1873 5a81b35b-ba03-0410-adc8-b2c5c5119f08