Improve inkscape_generate_png_from_svg

cmake/modules/UseInkscape.cmake

@@ -36,16 +36,28 @@ function(inkscape_generate_png_from_svg)
     set(INKSCAPE_DPI 90)
   endif()
 
+  message(STATUS "Generate PNG from SVG in ${INKSCAPE_OUTPUT_DIR}")
+  set(_converted 0)
   foreach(pic ${INKSCAPE_UNPARSED_ARGUMENTS})
     string(REGEX REPLACE "\\.[a-zA-Z]+" ".svg" input ${pic})
-    if( INKSCAPE_NEW_VERSION )
-      execute_process(
-        COMMAND ${INKSCAPE} --export-dpi=${INKSCAPE_DPI} --export-type=png --export-filename=${pic} ${CMAKE_CURRENT_SOURCE_DIR}/${input}
-        WORKING_DIRECTORY  ${INKSCAPE_OUTPUT_DIR})
-    else()
-      execute_process(
-        COMMAND ${INKSCAPE} -z --export-dpi=${INKSCAPE_DPI} -e ${pic} ${CMAKE_CURRENT_SOURCE_DIR}/${input}
-        WORKING_DIRECTORY  ${INKSCAPE_OUTPUT_DIR})
+    if(NOT EXISTS ${INKSCAPE_OUTPUT_DIR}/${pic})
+      set(_converted 1)
+      if(INKSCAPE_NEW_VERSION)
+        execute_process(
+          COMMAND ${INKSCAPE} --export-dpi=${INKSCAPE_DPI} --export-type=png --export-filename=${pic} ${CMAKE_CURRENT_SOURCE_DIR}/${input}
+          WORKING_DIRECTORY  ${INKSCAPE_OUTPUT_DIR}
+          OUTPUT_QUIET)
+      else()
+        execute_process(
+          COMMAND ${INKSCAPE} -z --export-dpi=${INKSCAPE_DPI} -e ${pic} ${CMAKE_CURRENT_SOURCE_DIR}/${input}
+          WORKING_DIRECTORY  ${INKSCAPE_OUTPUT_DIR}
+          OUTPUT_QUIET)
+      endif()
     endif()
   endforeach()
+  if(_converted)
+    message(STATUS "Generate PNG from SVG in ${INKSCAPE_OUTPUT_DIR} - done")
+  else()
+    message(STATUS "Generate PNG from SVG in ${INKSCAPE_OUTPUT_DIR} - skipped")
+  endif()
 endfunction()