lib: Makefile: Always use cmd.exe as SHELL on Windows

diff --git a/rpp/lib b/rpp/lib
index ca0de29090df4b18b57be0ad24e158087295c46b..23142fa52d828fccebe3eb651a787564c9c78fbe 160000 (submodule)
--- a/rpp/lib
+++ b/rpp/lib
@@ -1 +1 @@
-Subproject commit ca0de29090df4b18b57be0ad24e158087295c46b
+Subproject commit 23142fa52d828fccebe3eb651a787564c9c78fbe