make lftp usage configurable in env

diff --git a/builder.sh b/builder.sh
index 23e1822f80523722b815c8236ac461cbbe1ea263..e903fc6dc1c3ab3aebf4cdbbf7400da4c68aae0a 100755 (executable)
--- a/builder.sh
+++ b/builder.sh
@@ -114,8 +114,7 @@ PROTOCOL="http"
 IPOPT=""
 
 # use lftp by default when available
-USE_LFTP=
-lftp --version > /dev/null 2>&1 && USE_LFTP=yes
+test -z "${USE_LFTP+x}" && lftp --version > /dev/null 2>&1 && USE_LFTP=yes
 PARALLEL_DOWNLOADS=10
 
 WGET_RETRIES=${MAX_WGET_RETRIES:-0}