--- a/vcl/Executable_icontest.mk	
+++ a/vcl/Executable_icontest.mk	
@@ -19,6 +19,12 @@ $(eval $(call gb_Executable_use_externals,icontest,\
 ))
 endif
 
+ifeq ($(SYSTEM_GLM),TRUE)
+$(eval $(call gb_Executable_add_defs,icontest,\
+    -DGLM_ENABLE_EXPERIMENTAL \
+))
+endif
+
 $(eval $(call gb_Executable_use_api,icontest,\
     offapi \
     udkapi \