Introduce function that checks if the computations can be done on a GPU

Introduce function that checks if the computations can be done on a GPU

The following conditions should be fulfilled in order to compute on a GPU:
1. The code should be compiled with GPU support.
2. There should be compatible GPUs installed in the system.
3. These GPUs shuold be detectible from the code.
4. Environmental variable GMX_DISABLE_GPU_DETECTION should not be set.

TODO: This function is used in tests, so it should be eventually moved
      to src/testutils.

Change-Id: I40ce381b68fcebcfab330db4b48a1bec2a2b7a57