visibility warnings at link time

had loads of warning concerning different visibility such as:

ld: warning: __gnu_cxx::__normal_iterator, std::allocator >*, std::vector, std::allocator >, std::allocator, std::allocator > > > >::__normal_iterator(std::basic_string, std::allocator >* const&)has different visibility (default) in /builds/Debug-iphonesimulator/libX3N0_XPlat_Basic.a(X3N0_URLExtractor.o) and (hidden) in /builds/X3N0_iPhone_Band.build/Debug-iphonesimulator/X3N0_iPhone_Band_SOMETARGER.build/Objects-normal/i386/X3N0_MusicPlayerViewController.o

on xcode:

set "Symbols hidden by default" - GCC_SYMBOLS_PRIVATE_EXTERN to YES