configure: add --disable-shm to disable use of shm

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/configure b/configure
index 172b235..6e1376f 100755
--- a/configure
+++ b/configure
@@ -165,6 +165,8 @@
   ;;
   --enable-libhdfs) libhdfs="yes"
   ;;
+  --disable-shm) output_sym "CONFIG_NO_SHM"
+  ;;
   --help)
     show_help="yes"
     ;;