--- /dev/null
+--- a/drivers/can_socket/Makefile.in
++++ b/drivers/can_socket/Makefile.in
+@@ -23,7 +23,7 @@
+
+ CC = SUB_CC
+ OPT_CFLAGS = -O2
+-CFLAGS = SUB_OPT_CFLAGS
++CFLAGS ?= SUB_OPT_CFLAGS
+ PROG_CFLAGS = SUB_PROG_CFLAGS
+ PREFIX = SUB_PREFIX
+ TARGET = SUB_TARGET
+--- a/src/Makefile.in
++++ b/src/Makefile.in
+@@ -101,7 +101,7 @@ uninstall:
+ rm -rf $(DESTDIR)$(PREFIX)/include/canfestival
+
+ else
+-CFLAGS = SUB_OPT_CFLAGS
++CFLAGS ?= SUB_OPT_CFLAGS
+
+ ifeq ($(TARGET),none)
+ canfestival: libcanfestival.o
+--- a/drivers/unix/Makefile.in
++++ b/drivers/unix/Makefile.in
+@@ -85,7 +85,7 @@ install:
+
+
+ else
+-CFLAGS = SUB_OPT_CFLAGS
++CFLAGS ?= SUB_OPT_CFLAGS
+
+ driver: libcanfestival_$(TARGET).a
+
+--- a/drivers/timers_unix/Makefile.in
++++ b/drivers/timers_unix/Makefile.in
+@@ -23,7 +23,7 @@
+
+ CC = SUB_CC
+ OPT_CFLAGS = -O2
+-CFLAGS = SUB_OPT_CFLAGS
++CFLAGS ?= SUB_OPT_CFLAGS
+ PROG_CFLAGS = SUB_PROG_CFLAGS
+ OS_NAME = SUB_OS_NAME
+ ARCH_NAME = SUB_ARCH_NAME