[oct23av] in ggtt.mad cudacpp.mk, add override on top of ':=' to ensu…

…re that CUDACPP_MAKEFILE is an internal variable that cannot be set externally madgraph5#787

make -f cudacpp.mk cleanall
  CUDACPP_MAKEFILE='cudacpp.mk'
  CUDACPP_MAKEFILE='cudacpp.mk'
  cudacpp.mk:43: *** exit.  Stop.

This is now insensitive to variables coming from outside

make -f cudacpp.mk cleanall CUDACPP_MAKEFILE=pippo
  CUDACPP_MAKEFILE='cudacpp.mk'
  CUDACPP_MAKEFILE='cudacpp.mk'
  cudacpp.mk:43: *** exit.  Stop.

epochX/cudacpp/gg_tt.mad/SubProcesses/cudacpp.mk

@@ -7,7 +7,7 @@
 #=== NB: different names (e.g. cudacpp.mk and cudacpp_src.mk) are used in the Subprocess and src directories
 #=== NB: use ':=' to ensure that the value is not modified further down after including make_opts
 
-CUDACPP_MAKEFILE := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST))
+override CUDACPP_MAKEFILE := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST))
 $(info CUDACPP_MAKEFILE='$(CUDACPP_MAKEFILE)')
 
 CUDACPP_SRC_MAKEFILE = cudacpp_src.mk