Auto merge of #2018 - asomers:aio_no_librt, r=JohnTitor

aio functions do not require librt on FreeBSD

On FreeBSD, the aio_ functions require librt _only_ if they use
SIGEV_THREAD completion notification.  However, due to Rust's originally
poor support for C unions, libc doesn't even expose some of the fields of
struct sigevent that SIGEV_THREAD requires.  Accordingly, there is no
need to link librt to programs using aio via libc.

This change partially reverts 8c23f77
from PR #1630 .

While I'm here, fix the linkage of lio_listio on DragonflyBSD.  It
_does_ require librt.

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -1081,6 +1081,27 @@ extern "C" {
     ) -> *mut ::c_void;
 }
 
+#[link(name = "rt")]
+extern "C" {
+    pub fn aio_cancel(fd: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_error(aiocbp: *const aiocb) -> ::c_int;
+    pub fn aio_fsync(op: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_read(aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_return(aiocbp: *mut aiocb) -> ::ssize_t;
+    pub fn aio_suspend(
+        aiocb_list: *const *const aiocb,
+        nitems: ::c_int,
+        timeout: *const ::timespec,
+    ) -> ::c_int;
+    pub fn aio_write(aiocbp: *mut aiocb) -> ::c_int;
+    pub fn lio_listio(
+        mode: ::c_int,
+        aiocb_list: *const *mut aiocb,
+        nitems: ::c_int,
+        sevp: *mut sigevent,
+    ) -> ::c_int;
+}
+
 cfg_if! {
     if #[cfg(libc_thread_local)] {
         mod errno;

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -1210,6 +1210,18 @@ safe_f! {
 extern "C" {
     pub fn __error() -> *mut ::c_int;
 
+    pub fn aio_cancel(fd: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_error(aiocbp: *const aiocb) -> ::c_int;
+    pub fn aio_fsync(op: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_read(aiocbp: *mut aiocb) -> ::c_int;
+    pub fn aio_return(aiocbp: *mut aiocb) -> ::ssize_t;
+    pub fn aio_suspend(
+        aiocb_list: *const *const aiocb,
+        nitems: ::c_int,
+        timeout: *const ::timespec,
+    ) -> ::c_int;
+    pub fn aio_write(aiocbp: *mut aiocb) -> ::c_int;
+
     pub fn extattr_delete_fd(
         fd: ::c_int,
         attrnamespace: ::c_int,
@@ -1300,6 +1312,13 @@ extern "C" {
         flags: ::c_int,
     ) -> ::c_int;
 
+    pub fn lio_listio(
+        mode: ::c_int,
+        aiocb_list: *const *mut aiocb,
+        nitems: ::c_int,
+        sevp: *mut sigevent,
+    ) -> ::c_int;
+
     pub fn posix_fallocate(
         fd: ::c_int,
         offset: ::off_t,

src/unix/bsd/freebsdlike/mod.rs

@@ -1393,12 +1393,6 @@ extern "C" {
         timeout: *const ::timespec,
     ) -> ::c_int;
     pub fn lchflags(path: *const ::c_char, flags: ::c_ulong) -> ::c_int;
-    pub fn lio_listio(
-        mode: ::c_int,
-        aiocb_list: *const *mut aiocb,
-        nitems: ::c_int,
-        sevp: *mut sigevent,
-    ) -> ::c_int;
     pub fn lutimes(file: *const ::c_char, times: *const ::timeval) -> ::c_int;
     pub fn memrchr(
         cx: *const ::c_void,
@@ -1603,17 +1597,6 @@ extern "C" {
 
 #[link(name = "rt")]
 extern "C" {
-    pub fn aio_read(aiocbp: *mut aiocb) -> ::c_int;
-    pub fn aio_write(aiocbp: *mut aiocb) -> ::c_int;
-    pub fn aio_fsync(op: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
-    pub fn aio_error(aiocbp: *const aiocb) -> ::c_int;
-    pub fn aio_return(aiocbp: *mut aiocb) -> ::ssize_t;
-    pub fn aio_suspend(
-        aiocb_list: *const *const aiocb,
-        nitems: ::c_int,
-        timeout: *const ::timespec,
-    ) -> ::c_int;
-    pub fn aio_cancel(fd: ::c_int, aiocbp: *mut aiocb) -> ::c_int;
     pub fn mq_close(mqd: ::mqd_t) -> ::c_int;
     pub fn mq_getattr(mqd: ::mqd_t, attr: *mut ::mq_attr) -> ::c_int;
     pub fn mq_notify(mqd: ::mqd_t, notification: *const ::sigevent)