--- a/Makefile.in
+++ b/Makefile.in
@@ -9,6 +9,7 @@
 LDFLAGS+= @LDFLAGS@
 LIBS+= @LIBS@
 CFLAGS+= @CFLAGS@ @DEFS@ -W -Wall
+CPPFLAGS+= @CPPFLAGS@
 CPPFLAGS+=-DIPCAD_VERSION=\"@IPCAD_VERSION@\"
 CPPFLAGS+=-DCONFIG_FILE=\"${sysconfdir}/ipcad.conf\"
 CPPFLAGS+=@DEFS@ -D_REENTRANT -D_THREAD_SAFE
