Only remove ignored files in make clean

[crazytonyli]

My main concern with the current make clean command is it would remove untracked files. That would be quite devastating if I have untracked new source files and they are gone after a make clean re-build.

This PR replaces the -x option with -X.

       -x
           Don’t use the standard ignore rules (see gitignore(5)), but still use the ignore rules given with -e options from the command line. This
           allows removing all untracked files, including build products. This can be used (possibly in conjunction with git restore or git reset) to
           create a pristine working directory to test a clean build.

       -X
           Remove only files ignored by Git. This may be useful to rebuild everything from scratch, but keep manually created files.