- 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/README.md

@@ -0,0 +1,17 @@
+# Vendored Eigen 3.4.0 headers
+
+Eigen 3.4.0 (libeigen/eigen at commit
+3147391d946bb4b6c68edd901f2add6ac1f31f8c - the exact release libCZI pins in
+its cmake/ExternalEIGEN3.cmake) is vendored here, header-only, so that
+libczirw-sys builds need neither network access during the build (libCZI's
+default Eigen ExternalProject clone from gitlab.com is flaky in CI) nor a
+system libeigen3-dev package.
+
+Only the Eigen/ core-module tree is vendored - libCZI only does
+#include <Eigen/Eigen>. COPYING.MPL2 and COPYING.README are the
+upstream license files (Eigen defaults to MPL-2.0, with LGPL-3.0/GPL-3.0 as
+alternatives).
+
+Eigen3Config.cmake in this directory provides the Eigen3::Eigen interface
+target for CMake consumers using find_package(Eigen3 ...) (see
+LIBCZI_BUILD_PREFER_EXTERNALPACKAGE_EIGEN3 in the libCZI build).