[libglnx] Add missing configure check for copy_file_range

[Will Thompson <wjt src gnome org>]

commit 4577dc826a923d976ad1a98b4ed5ecc36eac6efe
Author: Will Thompson <wjt endlessm com>
Date:   Thu Nov 2 14:55:17 2017 +0000

    Add missing configure check for copy_file_range
    
    Without this, including glnx-missing-syscall.h raises this warning:
    
        ../ext/libglnx/glnx-missing-syscall.h:121:6: warning: "HAVE_DECL_COPY_FILE_RANGE" is not defined 
[-Wundef]
         #if !HAVE_DECL_COPY_FILE_RANGE
              ^~~~~~~~~~~~~~~~~~~~~~~~~

 libglnx.m4 |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/libglnx.m4 b/libglnx.m4
index 770f117..d5bcc2f 100644
--- a/libglnx.m4
+++ b/libglnx.m4
@@ -1,6 +1,9 @@
 AC_DEFUN([LIBGLNX_CONFIGURE],
 [
-AC_CHECK_DECLS([renameat2, memfd_create],
+AC_CHECK_DECLS([
+        renameat2,
+        memfd_create,
+        copy_file_range],
         [], [], [[
 #include <sys/types.h>
 #include <unistd.h>