[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, 1 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index dbf0999ce..6d74709a3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -538,6 +538,7 @@ if (NOT CMAKE_VERSION VERSION_LESS 3.0)
 
   # Imported target support
   add_library (eigen INTERFACE)
+  add_library (Eigen3::Eigen ALIAS eigen)
 
   target_compile_definitions (eigen INTERFACE ${EIGEN_DEFINITIONS})
   target_include_directories (eigen INTERFACE