Fails in UTF-8 locales with grep-2.6

https://bugs.gentoo.org/322151
https://savannah.gnu.org/bugs/index.php?31068

--- a/makeinfo/tests/accentenc
+++ b/makeinfo/tests/accentenc
@@ -1,7 +1,8 @@
 #!/bin/sh
 # Test encoded accent info output.
 
-unset TEXINFO_OUTPUT
+unset TEXINFO_OUTPUT LANG LANGUAGE
+LC_ALL=POSIX; export LC_ALL
 : ${srcdir=.}
 input=`basename $0`.txi
 output=`basename $0`.info