diff --git a/extensions/GNUmakefile.in b/extensions/GNUmakefile.in
index 14e7c57..c5d8844 100644
--- a/extensions/GNUmakefile.in
+++ b/extensions/GNUmakefile.in
@@ -21,7 +21,7 @@
 kinclude_CPPFLAGS  = @kinclude_CPPFLAGS@
 
 AM_CFLAGS       = ${regular_CFLAGS}
-AM_CPPFLAGS     = ${regular_CPPFLAGS} -I${top_builddir}/include -I${top_builddir} -I${top_srcdir}/include ${kinclude_CPPFLAGS}
+AM_CPPFLAGS     = ${regular_CPPFLAGS} -I${top_builddir}/include -I${top_builddir} -I${top_srcdir}/include ${kinclude_CPPFLAGS} ${CPPFLAGS}
 AM_DEPFLAGS     = -Wp,-MMD,$(@D)/.$(@F).d,-MT,$@
 AM_LDFLAGS      = @noundef_LDFLAGS@