DRY up Makefile using automatic variables

see https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html

Makefile

@@ -1,25 +1,28 @@
 .PHONY: lint check_dupes install
+
+sources :=$(wildcard ./exercises/**.json)
+
 lint:
-		check-jsonschema --schemafile ./schema.json ./exercises/**.json
+		check-jsonschema --schemafile ./schema.json $(sources)
 check_dupes:
 		# check for duplicate id's, if there's ID's listed here
 		# we've got duplicate id's that need to be resolved
-		jq -s ".[]" exercises/**.json | jq '.id' | sort | uniq -d
+		jq -s ".[]" $(sources) | jq '.id' | sort | uniq -d
 install:
 		pip install check-jsonschema
-dist/exercises.json: ./exercises/**.json
+dist/exercises.json: $(sources)
 		# requires jq
 		# brew install jq (for macos)
-		jq -s '.' ./exercises/**.json > ./dist/exercises.json
-dist/exercises.nd.json: ./exercises/**.json
+		jq -s '.' $^ > $@
+dist/exercises.nd.json: $(sources)
 		# output to new line delimited JSON
 		# for use to import into PostgreSQL via the COPY command
 		#
 	  # https://konbert.com/blog/import-json-into-postgres-using-copy
 		# https://www.postgresql.org/docs/current/sql-copy.html
-		jq -s '.[]' ./exercises/**.json > ./dist/exercises.nd.json
+		jq -s '.[]' $^ > $@
 dist/exercises.csv: dist/exercises.json
 		# output to csv format
 		# requires in2csv which is part of
 		# https://csvkit.readthedocs.io/
-		in2csv ./dist/exercises.json > ./dist/exercises.csv
+		in2csv ./dist/exercises.json > $@