[CMake] Phony target for version 3.0

[Kornel Benko]

Trying to use phony target (working with 2.8.12) like

	add_custom_target(doc0 DEPENDS ${some_dep_files})

leads to error messages like:

	CMake Error: Could not get source file entry for <builddir>/CMakeFiles/doc0

	Kornel

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