Right way to link library

Looks like Xcode ignore '-lc++' in CMAKE_CXX_LINK_FLAGS,
link can be done by 'target_link_libraries' command.
Actually it is the right way to do link.
Found by building Xcode project with shared libraries.
Tested configurations: shared/static + Xcode/make
Merge pull request #35 from ruslo/xcode.build

Right way to link library