Add USE_STAMP_FILE to doxygen_add_docs(...)

[ghost]

"If USE_STAMP_FILE is set, the custom command defined by this function will create a stamp file with the name <targetName>.stamp in the current binary directory whenever doxygen is re-run. With this option present, all items in <filesOrDirs> must be files (i.e. no directories, symlinks or wildcards) and each of the files must exist at the time doxygen_add_docs() is called. An error will be raised if any of the items listed is missing or is not a file when USE_STAMP_FILE is given. A dependency will be created on each of the files so that doxygen will only be re-run if one of the files is updated. Without the USE_STAMP_FILE option, doxygen will always be re-run if the <targetName> target is built regardless of whether anything listed in <filesOrDirs> has changed."

Try work around file/directory issue with GLOB commands.

[pjanck]

Not a priority, does not impede the development.