diff --git a/src/include/Makefile b/src/include/Makefile
index 7450231783..3a9b60aace 100644
--- a/src/include/Makefile
+++ b/src/include/Makefile
@@ -12,6 +12,7 @@ subdir = src/include
 top_builddir = ../..
 include $(top_builddir)/src/Makefile.global
 
+override CPPFLAGS := -I$(prefix)/include $(CPPFLAGS)
 
 all: pg_config.h pg_config_os.h check_oids check_keywords