tests: skip perl tests if NO_PERL is defined

These scripts all test git programs that are written in
perl, and thus obviously won't work if NO_PERL is defined.
We pass NO_PERL to the scripts from the building Makefile
via the GIT-BUILD-OPTIONS file.

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

t/t9500-gitweb-standalone-no-errors.sh

@@ -65,6 +65,11 @@ gitweb_run () {
 
 . ./test-lib.sh
 
+if ! test_have_prereq PERL; then
+	say 'skipping gitweb tests, perl not available'
+	test_done
+fi
+
 perl -MEncode -e 'decode_utf8("", Encode::FB_CROAK)' >/dev/null 2>&1 || {
     say 'skipping gitweb tests, perl version is too old'
     test_done