Improve handling of the deployment target setting on Mac OS X.

Due to matching against hardcoded versions of Darwin in the configure script
the deployment target for recent versions (10.7 and later) of Mac OS X was being
incorrectly set to 10.3.  This diff makes several changes to the handling of the
deployment target setting on Mac OS X:

- The default deployment target is now set to the same version as the host
  system.  (This is more consistent with the default XCode behaviour.)

- Matching against hardcoded versions of Darwin in the configure script
  for this purpose has been removed.

- There is a new option for the configure script,
  `--with-macosx-deployment-target', that allows the user to control
  what the deployment target for use with Mercury is.

configure.ac:
	Don't determine the deployment target by matching against hardcoded
	versions of Darwin -- we had failed to update this for several
	versions.

	By default set the deployment target to be the same version as
	the host system.

	Add a new option for overriding the default setting of the
	deployment target.

	s/doesn't/does not/ in a spot in order to prevent syntax
	highlighting in vim from going stupid.

README.MacOS:
	Add a paragraph describing how the deployment target is set
	for Mercury and pointing the user towards Apple's documentation
	if they require further information.

NEWS:
	Announce the change to the default value of the deployment
	target setting.