CMake: use GCC dependency generation for DAPSS files

cmake/DAPSS.cmake

@@ -54,13 +54,12 @@ function(add_daps_target target_name)
             list(APPEND include_flags "-I${include_dir}")
         endforeach()
 
-        # Add current source directory to includes if it's not already there
-        if(source_dir)
-            list(APPEND include_flags "-I${source_dir}")
-        endif()
+        list(APPEND include_flags "-I${CMAKE_CURRENT_SOURCE_DIR}")
 
-        # Convert list to space-separated string
-        string(REPLACE ";" " " include_flags_str "${include_flags}")
+        # Add source subdirectory to includes when SOURCES lists a path prefix
+        if(source_dir)
+            list(APPEND include_flags "-I${CMAKE_CURRENT_SOURCE_DIR}/${source_dir}")
+        endif()
 
         # Find C compiler if not already set
         if(NOT CMAKE_C_COMPILER)
@@ -70,11 +69,20 @@ function(add_daps_target target_name)
             endif()
         endif()
 
-        # Create custom command to preprocess the file
+        set(depfile "${output_file}.d")
+
+        # Preprocess with -MD so #included .dapss files become build dependencies
+        # (e.g. body specs that #include ../elp_4k_usb_cam.dapss).
         add_custom_command(
             OUTPUT ${output_file}
-            COMMAND sh -c "\"${CMAKE_C_COMPILER}\" -E -P -x c ${include_flags_str} \"${CMAKE_CURRENT_SOURCE_DIR}/${source_file}\" > \"${output_file}\""
-            DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/${source_file}
+            COMMAND "${CMAKE_C_COMPILER}"
+                -E -P -x c
+                ${include_flags}
+                -MD -MQ "${output_file}" -MF "${depfile}"
+                -o "${output_file}"
+                "${CMAKE_CURRENT_SOURCE_DIR}/${source_file}"
+            DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/${source_file}"
+            DEPFILE "${depfile}"
             COMMENT "Preprocessing ${source_file} to ${base_name}.daps"
             VERBATIM
         )