perl-5.26.1-4: Add check for DISPLAY

Its annoying if we fail after 30 minutes because we don't have an X11
display for Tk. Add code to fail early.

perl-5.26.1-4.build.sh

@@ -33,6 +33,13 @@ mkdir -p $HOME
 
 exec </dev/null		# to many cpan modules might ask something
 
+# Tk test needs display
+#
+if [ -z "$DISPLAY" ]; then
+	echo "DISPLAY not set. Required for Tk tests." >&2
+	exit 1
+fi
+
 # If you hit errors and want to rerun from where you failed,
 # use "if false","fi" to avoid rebuilding everything before
 # the error.