diff --git a/Makefile b/Makefile
index b1129be..45e39ec 100644
--- a/Makefile
+++ b/Makefile
@@ -22,7 +22,7 @@ PREFIX ?= /usr
 LIB_INSTALL_DIR ?= $(PREFIX)/lib/$(TARGET_FILE)
 INC_INSTALL_DIR ?= $(PREFIX)/include/lib$(TARGET_FILE)
 
-INCLUDES := inc
+INCLUDES += inc
 INCLUDES += $(PREFIX)/include/
 
 ifneq "$(findstring $(MAKECMDGOALS), build_unit_test exec_unit_test coverage)" ""