diff --git a/configure b/configure
index baac353..c13f5f4 100755
--- a/configure
+++ b/configure
@@ -330,7 +330,9 @@ native_defaults()
 
 
 	# os
-	mb_native_os=`uname | tr '[:upper:]' '[:lower:]'`
+	if [ -z "$mb_native_os" ]; then
+		mb_native_os=`uname | tr '[:upper:]' '[:lower:]'`
+	fi
 
 	mb_native_os_sizeof_pointer=`$mb_native_cc -dM -E - < /dev/null \
 			| awk '$2 == "__SIZEOF_POINTER__" { print $3 }'`