ENH: Use ITK's provided Eigen3Config.cmake to find Eigen

ITK's module Eigen require an itkeigen prefix to the paths and is not
compatible with a system Eigen library without the use of a
macro. This patch now restore the use of the vendored Eigen library
from the alternative package interface.

Author: blowekamp

CMakeLists.txt

@@ -46,21 +46,43 @@ else() # build proxTV here with the selected Eigen3
 #    set(CMAKE_CXX_EXTENSIONS OFF)
   endif()
 
-  set(proxTV_Eigen_LIBRARIES ITK::ITKEigen3Module)
-
   set(proxTV_GIT_REPOSITORY "https://github.com/InsightSoftwareConsortium/proxTV.git")
   set(proxTV_GIT_TAG "for/itk-proxtv-3.3.0-69062fe5")
+
   FetchContent_Declare(
     proxtv_fetch
     GIT_REPOSITORY ${proxTV_GIT_REPOSITORY}
     GIT_TAG ${proxTV_GIT_TAG}
      )
 
   # proxTV options:
+  message(STATUS "TotalVariation_proxTV_USE_EIGEN: ${TotalVariation_proxTV_USE_EIGEN}")
   if(TotalVariation_proxTV_USE_EIGEN)
     set(proxTV_USE_LAPACK 0)
+    if(NOT ITK_USE_SYSTEM_EIGEN)
+      # Set Eigen3_DIR to the internal Eigen3 used in ITK
+      set(Eigen3_DIR "${ITKInternalEigen3_DIR}")
+      message(STATUS "ITKTotalVariation: Using internal ITK Eigen Config found in: ${Eigen3_DIR}")
+    endif()
+    find_package(Eigen3 REQUIRED)
+    set(${PROJECT_NAME}_EXPORT_CODE_INSTALL
+  "${${PROJECT_NAME}_EXPORT_CODE_INSTALL}
+  find_package(OpenMP)
+  set(Eigen3_DIR \"${Eigen3_DIR}\")
+  find_package(Eigen3 REQUIRED CONFIG)
+  ")
+    set(${PROJECT_NAME}_EXPORT_CODE_BUILD
+  "${${PROJECT_NAME}_EXPORT_CODE_BUILD}
+  if(NOT ITK_BINARY_DIR)
+    find_package(OpenMP)
+    set(Eigen3_DIR \"${Eigen3_DIR}\")
+    find_package(Eigen3 REQUIRED CONFIG)
+  endif()
+  ")
   else()
+    # proxTV will be build with external system LAPACK instead of Eigen3
     set(proxTV_USE_LAPACK 1)
+    add_compile_definitions(PROXTV_USE_LAPACK)
   endif()
 
   # end proxTV options