[PATCH] cmake: Support source include with add_subdirectory and

 find_package use
This commit allows the sources of the project to be included in a parent
project CMakeLists.txt and support use of "find_package(Eigen3 CONFIG REQUIRED)"

Here is an example allowing to test the changes. It is not particularly
useful in itself. This change will allow to support one of the scenario
allowing to create custom 3D Slicer application bundling associated plugins.

/tmp/eigen-git-mirror  # Eigen sources

/tmp/test/CMakeLists.txt:

  cmake_minimum_required(VERSION 3.12)
  project(test)
  add_subdirectory("/tmp/eigen-git-mirror" "eigen-git-mirror")
  find_package(Eigen3 CONFIG REQUIRED)

and configuring it using:

  mkdir /tmp/test-build && cd $_
  cmake \
    -DCMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY:BOOL=1 \
    -DEigen3_DIR:PATH=/tmp/test-build/eigen-git-mirror \
    /tmp/test

Co-authored-by: Pablo Hernandez <pablo.hernandez@kitware.com>
---
 CMakeLists.txt              | 1 +
 cmake/Eigen3Config.cmake.in | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

1 files changed, 3 insertions, 1 deletions

diff --git a/cmake/Eigen3Config.cmake.in b/cmake/Eigen3Config.cmake.in
index c5c546887..0a1ac61c9 100644
--- a/cmake/Eigen3Config.cmake.in
+++ b/cmake/Eigen3Config.cmake.in
@@ -3,7 +3,9 @@
 
 @PACKAGE_INIT@
 
-include ("${CMAKE_CURRENT_LIST_DIR}/Eigen3Targets.cmake")
+if (NOT TARGET eigen)
+  include ("${CMAKE_CURRENT_LIST_DIR}/Eigen3Targets.cmake")
+endif ()
 
 # Legacy variables, do *not* use. May be removed in the future.