COMP: Address greptile review on vendored Eigen 5 import

Three findings on Modules/ThirdParty/Eigen3/src/itkeigen, all scoped to
the vendored Eigen build tree (USE_SYSTEM_EIGEN=ON consumers go through
find_package and never read these files; the system Eigen3Config.cmake
ships separately).

P1 — Eigen3Config.cmake.in:7 hardcoded "Eigen3Targets.cmake", dropping
the @EIGEN3_TARGETS_FILE@ substitution. The ITK section in
Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt sets
EIGEN3_TARGETS_FILE=ITKInternalEigen3Targets.cmake (line 972) before
calling configure_package_config_file against this template, but the
hardcoded form ignored that variable. The generated
ITKInternalEigen3Config.cmake would then include "…/Eigen3Targets.cmake"
(belonging to eigen_external), and ITKInternalEigen3::Eigen would
never be imported for installed consumers. Restore the substitution.

P2 — Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt declared
option(ITK_USE_EIGEN_MPL2_ONLY ...) twice in the ITK section: once at
line 844 and again at line 952. CMake silently ignores subsequent
option() calls when the variable is cached, so behavior is unchanged,
but the duplicate (and a slightly different description) is confusing.
Drop the second declaration; replace with a one-line pointer back to
the canonical declaration site.

P2 — Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt:964 had a
commented-out generator expression
"# $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/..>". Eigen 5's
header tree is fully sourced from CMAKE_CURRENT_SOURCE_DIR (no
configured headers land in the binary dir for this module), so the
commented line was dead code. Remove it.

USE_SYSTEM_EIGEN compatibility for Eigen 3.3+ is preserved: all three
edits live under Modules/ThirdParty/Eigen3/src/itkeigen which is only
configured when USE_SYSTEM_EIGEN=OFF; the system-Eigen path
(find_package(Eigen3 3.3 REQUIRED)) does not read these files.

Author: hjmjohnson

Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt

@@ -949,8 +949,7 @@ add_library (ITKInternalEigen3::Eigen ALIAS eigen_internal)
 # This would wrongly enforce EIGEN_MPL2_ONLY to other libraries using Eigen.
 # We wrap this definition in ITK_USE_EIGEN_MPL2_ONLY, and only enabling it internally in the dashboards and CI,
 # to avoid introducing GPL code from Eigen3 internally in ITK.
-option(ITK_USE_EIGEN_MPL2_ONLY "Set compile definition EIGEN_MPL2_ONLY for ITKInternalEigen3." OFF)
-mark_as_advanced(ITK_USE_EIGEN_MPL2_ONLY)
+# (ITK_USE_EIGEN_MPL2_ONLY is declared once near line 844 of this file.)
 
 if(ITK_USE_EIGEN_MPL2_ONLY)
   target_compile_definitions (eigen_internal INTERFACE "EIGEN_MPL2_ONLY")
@@ -961,7 +960,6 @@ endif()
 # INSTALL: headers require pre-prend itkeigen/Eigen/X.
 target_include_directories (eigen_internal SYSTEM INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
-  # $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/..>
   $<INSTALL_INTERFACE:${INCLUDE_INSTALL_DIR}>
   )
 

Modules/ThirdParty/Eigen3/src/itkeigen/cmake/Eigen3Config.cmake.in

@@ -4,5 +4,5 @@
 @PACKAGE_INIT@
 
 if (NOT TARGET Eigen3::Eigen)
-  include ("${CMAKE_CURRENT_LIST_DIR}/Eigen3Targets.cmake")
+  include ("${CMAKE_CURRENT_LIST_DIR}/@EIGEN3_TARGETS_FILE@")
 endif (NOT TARGET Eigen3::Eigen)