ioq: Check for io_uring_register_iowq_max_workers() before using it

This allows us to build against liburing as old as version 2.0, which is
apparently shipped by the latest Amazon Linux distribution.

Fixes: #147

build/has/io-uring-max-workers.c

@@ -0,0 +1,11 @@
+// Copyright © Tavian Barnes <[email protected]>
+// SPDX-License-Identifier: 0BSD
+
+#include <liburing.h>
+
+int main(void) {
+	struct io_uring ring;
+	io_uring_queue_init(1, &ring, 0);
+	unsigned int values[] = {0, 0};
+	return io_uring_register_iowq_max_workers(&ring, values);
+}

build/header.mk

@@ -33,6 +33,7 @@ HEADERS := \
     gen/has/getmntinfo.h \
     gen/has/getprogname-gnu.h \
     gen/has/getprogname.h \
+    gen/has/io-uring-max-workers.h \
     gen/has/pipe2.h \
     gen/has/posix-getdents.h \
     gen/has/posix-spawn-addfchdir-np.h \

src/ioq.c

@@ -878,6 +878,7 @@ static int ioq_ring_init(struct ioq *ioq, struct ioq_thread *thread) {
 		return -1;
 	}
 
+#if BFS_HAS_IO_URING_MAX_WORKERS
 	// Limit the number of io_uring workers
 	unsigned int values[] = {
 		ioq->nthreads, // [IO_WQ_BOUND]
@@ -886,6 +887,8 @@ static int ioq_ring_init(struct ioq *ioq, struct ioq_thread *thread) {
 	io_uring_register_iowq_max_workers(&thread->ring, values);
 #endif
 
+#endif // BFS_WITH_LIBURING
+
 	return 0;
 }