doctest should fail gracefully if GHCi is not supported

Hi,

we build and test packages on architecture where GHC does not support the interactive mode. There, doctest is failing, e.g.

```
Running 2 test suites...
Test suite doctests: RUNNING...
<command line>: not built for interactive use
doctests: fd:11: hGetLine: end of file
Test suite doctests: FAIL
Test suite logged to: dist-ghc/test/conduit-0.5.6-doctests.log
Test suite test: RUNNING...
Test suite test: PASS
Test suite logged to: dist-ghc/test/conduit-0.5.6-test.log
1 of 2 test suites (1 of 2 test cases) passed.
```

It would be great if doctest would detect that there is no interactive runtime available and then simply do nothing (besides possibly printing a warning).

This is related to http://bugs.debian.org/701059