Add compiler:c++11-lang to USES to fix build on GCC-based systems:
/usr/local/lib/libtcmalloc_minimal.so: undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::reserve(unsigned long)@GLIBCXX_3.4.21'
Approved by: portmgr (tier-2 blanket)