don't clutter output with install information if cmake is not a top-level project

Erik Schultheisrequested to merge
ngc92/eigen:cmake-34 into 3.4

This extracts a small piece of 19cacd3e to not print the installation instructions if Eigen is not used as a top-level project.

Contrary to that commit, this change does not try to fill in that information automatically based on the directories. So the change will mostly have an effect if used with very recent CMake (3.21 -- current ubuntu 21.10 uses 3.18), or if the PROJECT_IS_TOP_LEVEL variable is explicitly set by the user.

This potentially breaks a workflow that parses the cmake output of configuring Eigen and uses the installation instructions in some way, if used with the newest cmakes. Note that the information about the build, such as which libraries where found, the version of Eigen, cpu architecture, etc, is unaffected by this change.

Edited by Erik Schultheis

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