test failure on AIX

[Bruno Haible]

On AIX 7.2, the m4-1.4.18 build fails in "make check":

Checking ../../checks/006.command_li
@ ../doc/m4.texi:1000: Origin of test
../../checks/006.command_li: stderr mismatch
--- m4-tmp.23068986/m4-xerr     2020-05-03 06:30:35.000000000 -0500
+++ m4-tmp.23068986/m4-err      2020-05-03 06:30:35.000000000 -0500
@@ -1,2 +1,3 @@
+ sh: Broken pipe
  hi
  bye

...
Failed checks were:
  ../../checks/006.command_li:err

I reproduce with a probability of ca. 30% like this:

$ cat > in <<EOF
ifdef(`__unix__', ,
      `errprint(` skipping: syscmd does not have unix semantics
')m4exit(`77')')dnl
syscmd(`echo | cat >&- 2>/dev/null')ifelse(sysval, `0',
       `errprint(` skipping: system does not allow closing stdout
')m4exit(`77')')dnl
changequote(`[', `]')dnl
syscmd([echo 'esyscmd(echo hi >&2 && echo err"print(bye
)d"nl)dnl' > tmp.m4 \
  && ']__program__[' tmp.m4 <&- >&- \
  && rm tmp.m4])sysval
EOF

$ src/m4 < in
sh: There is no process to read data written to a pipe.
hi
bye
0

The attached patch fixes it.

0003-Fix-failure-of-test-006.command_li-on-AIX-7.2.patch
Description: Text Data