Style: changing preprocessor directives.

Using #if directives instead of #ifdef the same way as in other places.

src/nxt_conn_write.c

@@ -246,24 +246,30 @@ nxt_sendfile(int fd, int s, off_t pos, size_t size)
 {
     ssize_t  res;
 
-#ifdef NXT_HAVE_MACOSX_SENDFILE
+#if (NXT_HAVE_MACOSX_SENDFILE)
+
     off_t sent = size;
 
     int rc = sendfile(fd, s, pos, &sent, NULL, 0);
 
     res = (rc == 0 || sent > 0) ? sent : -1;
-#endif
 
-#ifdef NXT_HAVE_FREEBSD_SENDFILE
+#elif (NXT_HAVE_FREEBSD_SENDFILE)
+
     off_t sent = 0;
 
     int rc = sendfile(fd, s, pos, size, NULL, &sent, 0);
 
     res = (rc == 0 || sent > 0) ? sent : -1;
-#endif
 
-#ifdef NXT_HAVE_LINUX_SENDFILE
+#elif (NXT_HAVE_LINUX_SENDFILE)
+
     res = sendfile(s, fd, &pos, size);
+
+#else
+
+    res = -1;
+
 #endif
 
     return res;