makefile: don't re-generate if unchanged

Use make targets to skip "go generate" invocations when unnecessary.

Makefile

@@ -4,9 +4,13 @@ include ${SRHT_PATH}/Makefile
 
 all: api gitsrht-dispatch gitsrht-keys gitsrht-shell gitsrht-update-hook
 
-api:
+api/loaders/*_gen.go: api/loaders/generate.go api/loaders/gen go.sum
 	cd api && go generate ./loaders
+
+api/graph/api/generated.go: api/graph/schema.graphqls api/graph/generate.go go.sum
 	cd api && go generate ./graph
+
+api: api/graph/api/generated.go api/loaders/*_gen.go
 	cd api && go build
 
 gitsrht-dispatch: