diff -up VTK-9.1.0/ThirdParty/eigen/vtkeigen/CMakeLists.txt.eigen3 VTK-9.1.0/ThirdParty/eigen/vtkeigen/CMakeLists.txt
--- VTK-9.1.0/ThirdParty/eigen/vtkeigen/CMakeLists.txt.eigen3	2021-11-04 13:48:20.000000000 -0600
+++ VTK-9.1.0/ThirdParty/eigen/vtkeigen/CMakeLists.txt	2025-12-03 21:34:28.475070319 -0700
@@ -486,7 +486,7 @@ endif ()
 
 foreach(var INCLUDE_INSTALL_DIR CMAKEPACKAGE_INSTALL_DIR PKGCONFIG_INSTALL_DIR)
   if(IS_ABSOLUTE "${${var}}")
-    message(FATAL_ERROR "${var} must be relative to CMAKE_PREFIX_PATH. Got: ${${var}}")
+    file(RELATIVE_PATH "${var}" "${CMAKE_INSTALL_PREFIX}" "${${var}}")
   endif()
 endforeach()