Adjust test to ignore gzip 1.8+ warnings.

--- automake-1.15/t/distcheck-no-prefix-or-srcdir-override.sh	2016-06-14 00:36:26.554218552 +0200
+++ automake-1.15/t/distcheck-no-prefix-or-srcdir-override.sh	2016-06-14 00:37:52.903157770 +0200
@@ -49,7 +49,11 @@ grep "cannot find sources.* in foobar" s
 
 ./configure
 run_make -E -O distcheck
-test ! -s stderr
+
+# Gzip 1.8+ emits warnings like "gzip: warning: GZIP environment
+# variable is deprecated"; filter them out.
+test `grep -v '^gzip: warning' stderr | wc -l` -eq 0
+
 # Sanity check: the flags have been actually seen.
 $PERL -e 'undef $/; $_ = <>; s/ \\\n/ /g; print;' <stdout >t
 grep '/configure .* --srcdir am-src'   t || exit 99