title: CMake find_package not working for Eigen?
body: <p>I'm currently developing a Kalman Filtering library using Eigen and I've successfully gotten it working on my development Mac. Now I'm trying to set it up with Travis CI and CMake is having trouble with finding the package. First I <code>sudo apt install libeigen3-dev</code> and then attempt to run cmake with the following configuration:</p>
  
  <pre><code>cmake_minimum_required(VERSION 3.0)
  project(KFilter VERSION 0.1.0)
  
  find_package (Eigen3 REQUIRED NO_MODULE)
  add_library(KFilter KFilter.cpp)
  target_link_libraries(KFilter Eigen3::Eigen)
  </code></pre>
  
  <p>This builds just fine on my Mac, but in Travis CI it errors out with the following:</p>
  
  <pre><code>CMake Error at CMakeLists.txt:5 (add_library):
    Target "KFilter" links to target "Eigen3::Eigen" but the target was not
    found.  Perhaps a find_package() call is missing for an IMPORTED target, or
    an ALIAS target is missing?
  </code></pre>
  
  <p>Why is would I be getting this error at line 5 when the find_package seems to be successful? I'm following <a href="https://eigen.tuxfamily.org/dox/TopicCMakeGuide.html" rel="noreferrer">this</a> guide from the Eigen website.</p>
  
  <p>Travis CI is running Ubuntu 16.04 with CMake 3.12 and the Eigen3 debian package, while my Mac is running CMake 3.13 with Eigen installed through homebrew. I'm really confused as to why CMake is behaving differently.</p>