diff --git a/Makefile.in b/Makefile.in
index 1d25869c8ea..a9befca294b 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -78,7 +78,7 @@ $(S)config.stamp : $(S)configure $(S)Makefile.in
 
 define DEF_SUBMODULE_VARS
 DEPS_$(1) =
-CFLAGS_$(1) =
+CFLAGS_$(1) = -O2
 DONE_$(1) = $$(B)src/$(1)/$(1).dummy
 DEPS_SUBMODULES += $(1)
 endef