only spawn threads as necessary

src/DepNode.cc

 		return;
 	}
 
-	build_cond_.wait(lock, [=] { return build_complete_; });
+	build_cond_.wait(lock, [this] { return build_complete_; });
 }
 
 void DepNode::writeCompDB(const std::string &outDir, compdb::Writer &w) {

src/parallel.cc

 static std::unique_ptr<Worker[]> workers;
 static Worker *head= nullptr;
 static int threads_running = 0;
+static int max_threads = 0;
+static int num_threads = 0;
 static int waiting = 0;
 
 static void workerFunc(Worker *w) {
 		return ParallelContext{};
 	}
 
+	max_threads = num;
 	workers.reset(new Worker[num]);
-	for (int i = 0; i < num; ++i) {
-		Worker *w = &workers[i];
-		w->next_worker = head;
-		head = w;
-
-		w->running = true;
-		w->thread = std::thread(workerFunc, w);
-	}
 
+	// Don't spawn threads; they're spawned as needed
 	return ParallelContext{};
 }
 
 
 	workers.reset();
 	head = nullptr;
+	num_threads = 0;
+	max_threads = 0;
 }
 
 void run(std::function<void(void)> func) {
 		return;
 	}
 
+	if (head == nullptr && num_threads < max_threads) {
+		Worker *w = &workers[num_threads++];
+		w->next_worker = head;
+		head = w;
+
+		w->running = true;
+		w->thread = std::thread(workerFunc, w);
+	}
+
 	waiting += 1;
 	workers_cond.wait(lock, [] { return head != nullptr; });
 	Worker *w = head;