Skip t5540-http-push test when USE_CURL_MULTI is undefined

When USE_CURL_MULTI is undefined, git http-push doesn't work, so it's
useless to test it.

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5540-http-push.sh

@@ -12,6 +12,13 @@ This test runs various sanity checks on http-push.'
 ROOT_PATH="$PWD"
 LIB_HTTPD_DAV=t
 
+if git http-push > /dev/null 2>&1 || [ $? -eq 128 ]
+then
+	say "skipping test, USE_CURL_MULTI is not defined"
+	test_done
+	exit
+fi
+
 . ../lib-httpd.sh
 
 if ! start_httpd >&3 2>&4