fix genfilelist

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

2018/.filelist

@@ -1,11 +1,11 @@
 2018/2018.tar.bz2
-2018/Makefile
-2018/README.md
 2018/.filelist
 2018/.gitignore
 2018/guidelines.txt
 2018/index.html
 2018/iocccsize.c
 2018/iocccsize.mk
+2018/Makefile
+2018/README.md
 2018/rules.txt
 2018/.year