diff --git a/src/glsl/Makefile b/src/glsl/Makefile
index 4c85af8906d..cbdd0f9a7a3 100644
--- a/src/glsl/Makefile
+++ b/src/glsl/Makefile
@@ -82,7 +82,7 @@ GLSL2_OBJECTS = \
 
 ### Basic defines ###
 
-DEFINES = \
+DEFINES += \
 	$(LIBRARY_DEFINES) \
 	$(API_DEFINES)