- 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/COPYING.README

@@ -0,0 +1,18 @@
+Eigen is primarily MPL2 licensed. See COPYING.MPL2 and these links:
+  http://www.mozilla.org/MPL/2.0/
+  http://www.mozilla.org/MPL/2.0/FAQ.html
+
+Some files contain third-party code under BSD or LGPL licenses, whence the other
+COPYING.* files here.
+
+All the LGPL code is either LGPL 2.1-only, or LGPL 2.1-or-later.
+For this reason, the COPYING.LGPL file contains the LGPL 2.1 text.
+
+If you want to guarantee that the Eigen code that you are #including is licensed
+under the MPL2 and possibly more permissive licenses (like BSD), #define this
+preprocessor symbol:
+  EIGEN_MPL2_ONLY
+For example, with most compilers, you could add this to your project CXXFLAGS:
+  -DEIGEN_MPL2_ONLY
+This will cause a compilation error to be generated if you #include any code that is
+LGPL licensed.