Merged in alainvaucher/eigen/find-module-imported-target (pull request PR-324)

In the CMake find module, define the Eigen imported target as when installing with CMake

* In the CMake find module, define the Eigen imported target

* Add quotes to the imported location, in case there are spaces in the path.

Approved-by: Alain Vaucher <acvaucher@gmail.com>

cmake/FindEigen3.cmake

diff --git a/cmake/FindEigen3.cmake b/cmake/FindEigen3.cmake
index 9e96978..657440b 100644
--- a/cmake/FindEigen3.cmake
+++ b/cmake/FindEigen3.cmake
@@ -10,6 +10,10 @@
 #  EIGEN3_INCLUDE_DIR - the eigen include directory
 #  EIGEN3_VERSION - eigen version
 #
+# and the following imported target:
+#
+#  Eigen3::Eigen - The header-only Eigen library
+#
 # This module reads hints about search locations from 
 # the following enviroment variables:
 #
@@ -95,3 +99,8 @@
 
 endif(EIGEN3_INCLUDE_DIR)
 
+if(EIGEN3_FOUND AND NOT TARGET Eigen3::Eigen)
+  add_library(Eigen3::Eigen INTERFACE IMPORTED)
+  set_target_properties(Eigen3::Eigen PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "${EIGEN3_INCLUDE_DIR}")
+endif()