Unbreak building on FreeBSD without file AIO.

It appeared that the NGX_HAVE_AIO_SENDFILE macro was defined regardless of
the "--with-file-aio" configure option and the NGX_HAVE_FILE_AIO macro.

Now they are related.

Additionally, fixed one macro.

auto/os/freebsd

diff --git a/auto/os/freebsd b/auto/os/freebsd
index 6aa823f..22ad403 100644
--- a/auto/os/freebsd
+++ b/auto/os/freebsd
@@ -44,10 +44,12 @@
     CORE_SRCS="$CORE_SRCS $FREEBSD_SENDFILE_SRCS"
 fi
 
-if [ $osreldate -gt 502103 ]; then
-    echo " + sendfile()'s SF_NODISKIO found"
+if [ $NGX_FILE_AIO = YES ]; then
+    if [ $osreldate -gt 502103 ]; then
+        echo " + sendfile()'s SF_NODISKIO found"
 
-    have=NGX_HAVE_AIO_SENDFILE . auto/have
+        have=NGX_HAVE_AIO_SENDFILE . auto/have
+    fi
 fi
 
 # POSIX semaphores

src/core/ngx_buf.h

diff --git a/src/core/ngx_buf.h b/src/core/ngx_buf.h
index 219894f..9ee1c58 100644
--- a/src/core/ngx_buf.h
+++ b/src/core/ngx_buf.h
@@ -94,7 +94,7 @@
     unsigned                     aio:1;
 
     ngx_output_chain_aio_pt      aio_handler;
-#if (NGX_HAVE_FILE_AIO)
+#if (NGX_HAVE_AIO_SENDFILE)
     ssize_t                    (*aio_preload)(ngx_buf_t *file);
 #endif
 #endif