Mark the pybind11 include directory SYSTEM

`pybind11/eval.h` calls `_Py_fopen_obj`, which Python 3.14 deprecates.
The plain `include_directories()` applied the core warning flags to
that header, so `-Werror` turned it into a build failure. `pybind11`
marks its own headers `SYSTEM` only when consumed through
`add_subdirectory()`, leaving a `find_package()` consumer such as this
one to do it itself.

`SYSTEM` silences the third-party header while keeping `-Werror` on our
own deprecated uses, which `-Wno-error=deprecated-declarations` would
have demoted to a warning. `numpy` a few lines below already gets the
same treatment for the same reason.

Author: yungyuc

CMakeLists.txt

@@ -278,7 +278,9 @@ endif()
 option(pybind11_path "pybind11 path")
 find_package(pybind11 2.12.0 REQUIRED PATHS ${pybind11_path})
 message(STATUS "pybind11_INCLUDE_DIRS: ${pybind11_INCLUDE_DIRS}")
-include_directories(${pybind11_INCLUDE_DIRS})
+# SYSTEM keeps the core's warning flags off pybind11's own headers, the same
+# reason numpy gets it below.
+include_directories(SYSTEM ${pybind11_INCLUDE_DIRS})
 
 set(NUMPY_INCLUDE_DIR "${Python_NumPy_INCLUDE_DIRS}")
 message(STATUS "NUMPY_INCLUDE_DIR: ${NUMPY_INCLUDE_DIR}")