From Stephan Huber, "while debugging a problem in finding plugins on OS X I discovered, that
the conditional directives for setting the prepend-string in
createLibraryNameForExtension were not in effect, because of the mixture
of different #ifdef styles.
I removed the conditional part for APPLE completely to be more
standard-conform with other platforms (plugins should be located in
osgPlugins-X.X.X/). Because of the wrong syntax of the conditional
compile the old code was not used anyway -- so no functional change.
"