Configure: fix build with -Werror=unused-value.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

auto/cc/conf

diff --git a/auto/cc/conf b/auto/cc/conf
index f2c25ed..b3b9f92 100644
--- a/auto/cc/conf
+++ b/auto/cc/conf
@@ -231,7 +231,7 @@
     ngx_feature_incs=
     ngx_feature_path=
     ngx_feature_libs=
-    ngx_feature_test="__builtin_bswap64(0)"
+    ngx_feature_test="if (__builtin_bswap64(0)) return 1"
     . auto/feature