fix: boost's include dir was listed first (#2384)

tests/CMakeLists.txt

@@ -236,13 +236,13 @@ foreach(target ${test_targets})
     if(PYBIND11_EIGEN_VIA_TARGET)
       target_link_libraries(${target} PRIVATE Eigen3::Eigen)
     else()
-      target_include_directories(${target} PRIVATE ${EIGEN3_INCLUDE_DIR})
+      target_include_directories(${target} SYSTEM PRIVATE ${EIGEN3_INCLUDE_DIR})
     endif()
     target_compile_definitions(${target} PRIVATE -DPYBIND11_TEST_EIGEN)
   endif()
 
   if(Boost_FOUND)
-    target_include_directories(${target} PRIVATE ${Boost_INCLUDE_DIRS})
+    target_include_directories(${target} SYSTEM PRIVATE ${Boost_INCLUDE_DIRS})
     target_compile_definitions(${target} PRIVATE -DPYBIND11_TEST_BOOST)
   endif()
 

tests/test_embed/CMakeLists.txt

@@ -14,7 +14,7 @@ else()
 endif()
 
 add_executable(test_embed catch.cpp test_interpreter.cpp)
-target_include_directories(test_embed PRIVATE "${CATCH_INCLUDE_DIR}")
+target_include_directories(test_embed SYSTEM PRIVATE "${CATCH_INCLUDE_DIR}")
 pybind11_enable_warnings(test_embed)
 
 target_link_libraries(test_embed PRIVATE pybind11::embed)