Enable "make package" and "make package_source"

[octave-user]

Dear Google Team,

With this pull request, it will be possible to run "make package_source" (e.g. googletest-distribution-0.1.1-Source.tar.gz to be included in a docker container):

$ make package_source
Run CPack packaging tool for source...
CPack: Create package using TGZ
CPack: Install projects
CPack: - Install directory: ~/install/googletest
CPack: Create package
CPack: - package: ~/install/googletest-build/googletest-distribution-0.1.1-Source.tar.gz generated.

Reinhard

[google-cla]

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