[UNIKRAFT PATCH 2/7] build: Use secondary expansion for M4 pre-processing

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Similar to our build rules, we enable secondary expansion of
flags and includes to M4 pre-processing rules.

Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
---
 support/build/Makefile.rules | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/support/build/Makefile.rules b/support/build/Makefile.rules
index 82d8fd7c..44701cd2 100644
--- a/support/build/Makefile.rules
+++ b/support/build/Makefile.rules
@@ -620,10 +620,10 @@ endef
 define preprule_m4 =
 $(3): $(2) | prepare
        $(call build_cmd,M4,$(1),$(3),\
-               $(M4)  $(M4INCLUDES) $(M4INCLUDES-y) \
-                      $($(call vprefix_lib,$(1),M4INCLUDES)) $($(call 
vprefix_lib,$(1),M4INCLUDES-y)) \
-                      $(M4FLAGS) $(M4FLAGS-y) \
-                      $($(call vprefix_lib,$(1),M4FLAGS)) $($(call 
vprefix_lib,$(1),M4FLAGS-y)) \
+               $(M4)  $$(M4INCLUDES) $$(M4INCLUDES-y) \
+                      $$($(call vprefix_lib,$(1),M4INCLUDES)) $$($(call 
vprefix_lib,$(1),M4INCLUDES-y)) \
+                      $$(M4FLAGS) $$(M4FLAGS-y) \
+                      $$($(call vprefix_lib,$(1),M4FLAGS)) $$($(call 
vprefix_lib,$(1),M4FLAGS-y)) \
                       $(4) \
                       $(2) > $(3)
        )
-- 
2.20.1