[osx] - fixed linkage of libxbmc.so, xbmc.bin and xbmc-test

As the title says.

See commit messages for further description.

This already passed jenkins testing.

This is needed for getting gtest hoooked up on jenkins ... another pr for this might follow soon ...

IMO this mainobjs thingy is something that should go into xbmc/main/Makefile.in and we use main.a instead - but somehow we mix xbmc/xbmc.o in here which would be a nogo in xbmc/main/Makefile.in. So i am not sure how to get rid of main.a here. All other gccs might support the --start-group/--end-group syntax which prevents duplicated symbols during linkage - but on osx it is not available and using MAINOBJS on osx would lead to dupe definition of xbmc.o.

So this mainly is a partly revert of what was a try to fix things (without paying attention to osx...).

For discussion (don't overdo this...) with the buildsystem pro's ...