Allow older process for compatibility

In [two](b6f49ef153036c9ddaeeb110a8207172e28af409) previous [commits](770c071cabd3588acf937ef8f2916ad1f51a505b), I replaced a deprecated function (`system`) with a newer one (`callCommand`) which requires `process-1.2.0.0`. While this works well on GHC 7.8.3, I inadvertently made it far more difficult to install `cab` on older GHCs which are bundled with `process-1.1` or earlier.

To atone for this mistake, I went back and added CPP pragmas so that (1) earlier versions of `process` can be used, and (2) if `process-1.2` is being used, use `callCommand` instead of `system`.