--- a/rp2gen/Makefile.am
+++ b/rp2gen/Makefile.am
@@ -3,13 +3,9 @@
 CCLD=$(NATIVECC)
 #EXEEXT=
 AM_YFLAGS= -d
-AM_CPPFLAGS= -I$(top_builddir)/include -I$(top_srcdir)/include -I$(srcdir) -I.
+AM_CPPFLAGS= -I$(top_builddir)/include -I$(top_srcdir)/include -I$(srcdir) -I. $(CFLAGS)
 
 # override any cross compilation target flags
-CFLAGS=-Wall
-LDFLAGS=
-LIBS=
-
 .c.o:
 	$(NATIVECC) $(AM_CPPFLAGS) -c $<