cmake: eliminate ninja generator warning (CMake 3.10), refactor code

modules/python/bindings/CMakeLists.txt

@@ -38,15 +38,12 @@ ocv_list_filterout(opencv_hdrs "modules/.*_inl\\\\.h*")
 ocv_list_filterout(opencv_hdrs "modules/.*\\\\.details\\\\.h*")
 ocv_list_filterout(opencv_hdrs "modules/.*/detection_based_tracker\\\\.hpp") # Conditional compilation
 
-set(cv2_generated_hdrs
+set(cv2_generated_files
     "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_generated_include.h"
     "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_generated_funcs.h"
     "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_generated_types.h"
     "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_generated_type_reg.h"
     "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_generated_ns_reg.h"
-)
-
-set(cv2_generated_files ${cv2_generated_hdrs}
     "${OPENCV_PYTHON_SIGNATURES_FILE}"
 )
 
@@ -54,18 +51,29 @@ string(REPLACE ";" "\n" opencv_hdrs_ "${opencv_hdrs}")
 file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/headers.txt" "${opencv_hdrs_}")
 add_custom_command(
     OUTPUT ${cv2_generated_files}
-    COMMAND ${PYTHON_DEFAULT_EXECUTABLE} "${PYTHON_SOURCE_DIR}/src2/gen2.py" ${CMAKE_CURRENT_BINARY_DIR} "${CMAKE_CURRENT_BINARY_DIR}/headers.txt"
-    DEPENDS ${PYTHON_SOURCE_DIR}/src2/gen2.py
-    DEPENDS ${PYTHON_SOURCE_DIR}/src2/hdr_parser.py
-    DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/headers.txt
-    DEPENDS ${opencv_hdrs}
+    COMMAND "${PYTHON_DEFAULT_EXECUTABLE}" "${PYTHON_SOURCE_DIR}/src2/gen2.py" "${CMAKE_CURRENT_BINARY_DIR}" "${CMAKE_CURRENT_BINARY_DIR}/headers.txt"
+    DEPENDS "${PYTHON_SOURCE_DIR}/src2/gen2.py"
+            "${PYTHON_SOURCE_DIR}/src2/hdr_parser.py"
+            # not a real build dependency (file(WRITE) result): ${CMAKE_CURRENT_BINARY_DIR}/headers.txt
+            ${opencv_hdrs}
     COMMENT "Generate files for Python bindings and documentation"
 )
 
 add_custom_target(gen_opencv_python_source DEPENDS ${cv2_generated_files})
 
 set(cv2_custom_hdr "${CMAKE_CURRENT_BINARY_DIR}/pyopencv_custom_headers.h")
-file(WRITE ${cv2_custom_hdr} "//user-defined headers\n")
+set(cv2_custom_hdr_str "//user-defined headers\n")
 foreach(uh ${opencv_userdef_hdrs})
-    file(APPEND ${cv2_custom_hdr} "#include \"${uh}\"\n")
+    set(cv2_custom_hdr_str "${cv2_custom_hdr_str}#include \"${uh}\"\n")
 endforeach(uh)
+if(EXISTS "${cv2_custom_hdr}")
+  file(READ "${cv2_custom_hdr}" __content)
+else()
+  set(__content "")
+endif()
+if("${__content}" STREQUAL "${cv2_custom_hdr_str}")
+  # Up-to-date
+else()
+  file(WRITE "${cv2_custom_hdr}" "${cv2_custom_hdr_str}")
+endif()
+unset(__content)