improve tab_check Makefile rule

We needed to use the proper make variables.

Makefile

@@ -317,9 +317,9 @@ help:
 # verify that there are no leading ASCII tabs in leading whitespace in markdown files
 tab_check:
 	@echo '=-=-=-=-= IOCCC begin make $@ =-=-=-=-='
-	@TAB_LIST="$$(find * \( -name NOTES -o -name tmp -o -name markdown.md \) -prune -o \
+	@TAB_LIST="$$(${FIND} * \( -name NOTES -o -name tmp -o -name markdown.md \) -prune -o \
 				     -type f -name '*.md' -print0 | \
-			      xargs -0 grep -E -l '\t')"; \
+			      ${XARGS} -0 ${GREP} -E -l '\t')"; \
 	    if [[ -n $$TAB_LIST ]]; then \
 	        echo "$@: ERROR: ASCII tab found in markdown file(s)" 1>&2 ; \
 		echo "$0: Warning: list of offending markdown file(s) starts below" 1>&2 ; \