nl_langinfo: new failure with glibc-2.26 and fedora 27

[Jim Meyering]

Hi Bruno,

I wanted to update sed to latest gnulib, but found that the new
nl_langinfo tests would fail on fedora 27 with glibc-2.26, where
nl_langinfo (ALTMON_1) returns the empty string:

FAIL: test-nl_langinfo.sh
=========================

test-nl_langinfo.c:95: assertion 'strlen (nl_langinfo (ALTMON_1)) > 0' failed
./test-nl_langinfo.sh: line 3: 10789 Aborted                 (core
dumped) LC_ALL=C ./test-nl_langinfo${EXEEXT} 0
FAIL test-nl_langinfo.sh (exit status: 1)