Port some of the generator API to cmGeneratorTarget.

Just enough to reach the BuildMacContentDirectory method and the
NeedRelinkBeforeInstall methods.

In the future, those methods can be moved to cmGeneratorTarget.
Add a null check to the generator target accessor.
Fix line length.
Merge topic 'use-generator-target' into next

e16de4b Add a null check to the generator target accessor.
29be12b Port some of the generator API to cmGeneratorTarget.
Merge topic 'use-generator-target' into next

d200b63 Fix line length.
Port some of the generator API to cmGeneratorTarget.

Just enough to reach the BuildMacContentDirectory method and the
NeedRelinkBeforeInstall methods.

In the future, those methods can be moved to cmGeneratorTarget.
Add a null check to the generator target accessor.
Merge topic 'use-generator-target' into next

cef4f54 Add a null check to the generator target accessor.
556aed4 Port some of the generator API to cmGeneratorTarget.