maint: adjust to Gnulib API changes

* src/output.c (output_skeleton):
* src/print-xml.c (print_html):
Adjust to Gnulib API changes.

src/output.c

@@ -808,6 +808,7 @@ output_skeleton (void)
       }
 
     pid = create_pipe_bidi ("m4", m4, argv,
+                            /* dll_dirs */ NULL,
                             /* directory */ NULL,
                             /* null_stderr */ false,
                             /* slave_process */ true,

src/print-xml.c

@@ -565,6 +565,7 @@ print_html (void)
   int status
     = execute (argv[0],
                argv[0], argv,
+               /* dll_dirs */ NULL,
                /* directory */ NULL,
                /* ignore_sigpipe */ false,
                /* null_stdin, null_stdout, null_stderr */ true, true, true,