Fixes #255

`checkcxx.cpp` builds successfully from cmake if c++11 is supported by the compiler or outputs an error message
Note : I did not check on OS X
We should probably add here all other C++ constructs that we use. E.g. things like:
```c++
    std::size_t seed = 0;
    hash_combine<Basic>(seed, *coef_);
    map_basic_num ordered(dict_.begin(), dict_.end());
    for (auto &p: ordered) {
        hash_combine<Basic>(seed, *(p.first));
        hash_combine<Basic>(seed, *(p.second));
    }
    return seed;
```
I would strip it from csympy dependence, just use a simple C++ constructs. Also we should keep the file as simple as possible. So that if it fails, people can just try to compile the it by hand with their compiler and figure out why it fails.

I don't know which features we should test. We can also just keep it as it is, and only when somebody posts a build failure, only then add more things here.