t7800: respect NO_PERL

Difftool is written in perl, so we don't build it if NO_PERL
is set.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

t/t7800-difftool.sh

@@ -10,6 +10,11 @@ Testing basic diff tool invocation
 
 . ./test-lib.sh
 
+if ! test_have_prereq PERL; then
+	say 'skipping difftool tests, perl not available'
+	test_done
+fi
+
 remove_config_vars()
 {
 	# Unset all config variables used by git-difftool