This allows for a built in bzip and zip capability, so external tools
will not be needed for these packagers.  The cmake -E tar xf should be
able to handle all compression types now as well.

As of CMake 2.6 this variable is not defined, and the ANSI flags for the
HP compiler are simply hard-coded in the default C flags.

It is useful to be able to test if a target has been created.  Often
targets are created only inside conditions.  Rather than storing the
result of the condition manually for testing by other parts of the
project, it is much easier for the other parts to just test for the
target's existence.  This will also be useful when find-modules start
reporting results with IMPORTED targets and projects want to test if a
certain target is available.