Make the Property name protected so that subclasses can use it.

Makes subclasses more dry in upcoming patches.
Don't allow targets args in the new target commands.
Make subclasses responsible for joining content.

This way we can add handling of relative/absolute paths and of
-D in compile definitions.
CMake Nightly Date Stamp
Merge topic 'fix-target-property-commands'

7bf490e Make subclasses responsible for joining content.
f6b16d4 Don't allow targets args in the new target commands.
b3a7e19 Make the Property name protected so that subclasses can use it.