Fix build on gcc-based architectures:
undefined reference to 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)@GLIBCXX_3.4.21'
Approved by: portmgr (tier-2 blanket)