diff --git a/sys/unix/Makefile.src b/sys/unix/Makefile.src
index 623d19c12..fac0403f7 100644
--- a/sys/unix/Makefile.src
+++ b/sys/unix/Makefile.src
@@ -188,6 +188,8 @@ SYSOBJ = $(TARGETPFX)ioctl.o $(TARGETPFX)unixmain.o $(TARGETPFX)unixtty.o \
 #CFLAGS = -O -I../include
 #LFLAGS =
 
+CFLAGS ?= -I../include
+
 AR = ar
 ARFLAGS = rcs
 
diff --git a/sys/unix/Makefile.utl b/sys/unix/Makefile.utl
index 6e8aa0c71..9ae37bdec 100644
--- a/sys/unix/Makefile.utl
+++ b/sys/unix/Makefile.utl
@@ -106,6 +106,8 @@ NHSROOT=..
 #CFLAGS = -O -I../include
 #LFLAGS =
 
+CFLAGS ?= -I../include
+
 # -lm required by lua
 LFLAGS += -lm