Configure: fixed autotest cleanup commands.

Previously, if configured with --with-cc="clang -g", the autotest.dSYM
directories were left unremoved.
diff --git a/auto/include b/auto/include
index 3466fda..e34dabd 100644
--- a/auto/include
+++ b/auto/include
@@ -58,4 +58,4 @@
     echo "----------"    >> $NGX_AUTOCONF_ERR
 fi
 
-rm $NGX_AUTOTEST*
+rm -rf $NGX_AUTOTEST*