Build break on Linux/CLang

[evoskuil]

CXXLD  bitcoin_server
src/bitcoin_server-config.o: In function `server::load_config(server::config_type&, boost::filesystem::path&)':
src/config.cpp:(.text+0x5b3): undefined reference to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const'
src/config.cpp:(.text+0x5c4): undefined reference to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const'
src/config.cpp:(.text+0x5d5): undefined reference to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const'
src/config.cpp:(.text+0x5e6): undefined reference to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const'
src/config.cpp:(.text+0x5f7): undefined reference to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const'
src/bitcoin_server-config.o:src/config.cpp:(.text+0x608): more undefined references to `libconfig::Setting::lookupValue(char const*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >&) const' follow
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [bitcoin_server] Error 1

OK on Linux/GCC, OSX/CLang and VC++.

[evoskuil]

This would be rendered moot by switching to boost::program_options, which we should do in any case.

[evoskuil]

This has been resolved by the change to stdlibc++ in linux/clang.