It's already defined in LibOS/shim/test/Makefile.Test.
@@ -8,8 +8,6 @@ include ../../../../Makefile.configs
include ../../../../Makefile.rules
include ../Makefile.Test
-CFLAGS-libos = -L../../../glibc-build/libos -I../../include
-
%.manifest: %.manifest.template
rm -rf $@
cp $@.template $@