Add sol.main.get_platform function. Fix https://github.com/christopho/so...

...larus/issues/562
CLang returns this without the c_str() : 
/usr/local/include/luajit-2.0/lua.h:164:16: note: candidate function not viable:
      no known conversion from 'const std::string' (aka 'const
      basic_string<char>') to 'const char *' for 2nd argument
LUA_API void  (lua_pushstring) (lua_State *L, const char *s);