MFH: r541485

audio/adplay: fix build on GCC architectures

Use C++11 compiler:
/usr/local/lib/libadplug.so: undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::append(char const*)@GLIBCXX_3.4.21'

Approved by: portmgr (fix build blanket)