configure: HPUX warning for empty CC

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/configure b/configure
index cc4bdeb..7352bf1 100755
--- a/configure
+++ b/configure
@@ -252,7 +252,7 @@
   ;;
 esac
 
-if test -z $CC; then
+if test -z "$CC" ; then
   if test "$targetos" = "FreeBSD"; then
     if has clang; then
       CC=clang