Fix m4 build with the newest gnulib

[Bruno Haible]

With the newest gnulib, a build of m4 from git fails:

make[2]: Entering directory '/tmp/m4/src'
  CC       m4.o
  CC       builtin.o
builtin.c: In function 'm4_syscmd':
builtin.c:968:44: warning: passing argument 3 of 'execute' from incompatible 
pointer type [-Wincompatible-pointer-types]
   status = execute (ARG (0), SYSCMD_SHELL, (char **) prog_args, false,
                                            ^
In file included from builtin.c:27:0:
../lib/execute.h:48:12: note: expected 'const char * const*' but argument is of 
type 'char **'
 extern int execute (const char *progname,
            ^
builtin.c:968:12: error: too few arguments to function 'execute'
   status = execute (ARG (0), SYSCMD_SHELL, (char **) prog_args, false,
            ^
In file included from builtin.c:27:0:
../lib/execute.h:48:12: note: declared here
 extern int execute (const char *progname,
            ^
builtin.c: In function 'm4_esyscmd':
builtin.c:1011:50: warning: passing argument 3 of 'create_pipe_in' from 
incompatible pointer type [-Wincompatible-pointer-types]
   child = create_pipe_in (ARG (0), SYSCMD_SHELL, (char **) prog_args,
                                                  ^
In file included from builtin.c:31:0:
../lib/spawn-pipe.h:113:14: note: expected 'const char * const*' but argument 
is of type 'char **'
 extern pid_t create_pipe_in (const char *progname,
              ^
builtin.c:1011:11: error: too few arguments to function 'create_pipe_in'
   child = create_pipe_in (ARG (0), SYSCMD_SHELL, (char **) prog_args,
           ^
In file included from builtin.c:31:0:
../lib/spawn-pipe.h:113:14: note: declared here
 extern pid_t create_pipe_in (const char *progname,
              ^
make[2]: *** [Makefile:1949: builtin.o] Error 1


This is due to the recent changes in the gnulib modules 'execute' and
'spawn-pipe' (see gnulib/NEWS). This patch fixes it. Can you please
apply it? Thanks.

0001-Update-after-gnulib-changed.patch
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