- make the build self-contained and cross-compile friendly: vendor the Eigen 3.4.0 headers (instead of the flaky gitlab clone / a system libeigen3-dev package), align the msvc C runtime with the dynamic one used by the rest of cross builds (xwin), and pass the -D_LIBCPP_* macros to bindgen so it parses zig's bundled libc++ regardless of ziglang/cargo-zigbuild versions

eigen/Eigen3Config.cmake

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: MPL-2.0
+#
+# Minimal CMake config for the vendored Eigen 3.4.0 headers.
+#
+# The Eigen 3.4.0 release does not ship pre-generated Eigen3Config.cmake /
+# Eigen3Targets.cmake (those only appear in distro packages), so provide the
+# Eigen3::Eigen interface target here for
+#   find_package(Eigen3 REQUIRED NO_MODULE)
+# consumers (libCZI, with LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_EIGEN3=ON).
+if(NOT TARGET Eigen3::Eigen)
+  get_filename_component(_eigen3_root "${CMAKE_CURRENT_LIST_DIR}" ABSOLUTE)
+  add_library(Eigen3::Eigen INTERFACE IMPORTED GLOBAL)
+  set_target_properties(Eigen3::Eigen PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "${_eigen3_root}")
+  set(EIGEN3_INCLUDE_DIR "${_eigen3_root}")
+endif()