region: Enable or disable fatal errors and selfchecks based on version number

There is a couple of bugs in bugzilla where bugs in the X server
triggered asserts in the pixman region code. It is probably better to
let the X server survive this. (In fact, I thought I had disabled them
for 0.16.0, but apparently not).

The patch below uses these rules:

    - In _stable_ pixman releases, assertions and selfchecks are turned
      off. Assertions, so that the X server doesn't die. Selfchecks,
      for performance reasons.

    - In _unstable_ pixman releases, both assertions and selfcheck are
      turned on. These releases are what get added to development
      distributions such as rawhide, so we want as much self-checking
      as possible.

    - In _random git checkouts_, assertions are enabled, so that bugs
      are caught, but selfchecks are disabled so that you can use them
      for performance work without having to fiddle with turning
      selfchecks off.

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