fix genfilelist

Fixed the sort for `make genfilelist` to ignore case
and to use the ${SORT} makefile variable.

2001/.filelist

@@ -1,10 +1,10 @@
 2001/2001.tar.bz2
-2001/Makefile
-2001/README.md
 2001/.filelist
 2001/.gitignore
 2001/guidelines.txt
 2001/index.html
+2001/Makefile
 2001/mkentry.c
+2001/README.md
 2001/rules.txt
 2001/.year