Overload cmLocalGenerator::AppendDefines to add a list.
Refactor cmTarget::GetCompileDefinitions to use an out-vector, not a string.
Add target property debugging for COMPILE_DEFINITIONS

Use constructs similar to those for COMPILE_OPTIONS. This is a little
different because there is a command to remove_definitions(), so
we can't populate the equivalent target property until generate-time
in cmGlobalGenerator.
Merge topic 'compile-defs-debugging' into next

3919a76 Add target property debugging for COMPILE_DEFINITIONS
00ff09b Refactor cmTarget::GetCompileDefinitions to use an out-vector, not a string.
963a0ba Overload cmLocalGenerator::AppendDefines to add a list.