cmTarget: Allow any generator expression in SOURCES property.
Remove use of UseObjectLibraries from Makefile and Ninja generators. It is not needed now because those generators use GetExternalObjects which already contains the objects from object libraries. The VS10 generator calls both the UseObjectLibraries and the GetExternalObjects methods. Ensure that duplicates are not created by skipping objects from object libraries in handling of GetExternalObjects. Similarly, fix VS6, VS7 and Xcode object handling by skipping external objects from OBJECT_LIBRARY usage as appropriate. The error message in the BadSourceExpression1 test is now reported by the generator expression evaluator, so it has different text.
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