You need --coverage when linking as well

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index e5dddba..5e0275a 100644
--- a/Makefile
+++ b/Makefile
@@ -100,8 +100,9 @@
 test: check
 
 ifndef WINDOWS
-# note: for coverage testing, build with:
-# make CFLAGS='--coverage -g3 -O0'
+# Note: for coverage testing, build with:
+#     make CFLAGS='--coverage -g3 -O0' LDFLAGS='--coverage -g3 -O0'
+# For the reference coverage measurement, see tests/scripts/basic-build-test.sh
 covtest:
 	$(MAKE) check
 	programs/test/selftest