diff --git a/tests/Makefile b/tests/Makefile
index 7b57327..39dce5b 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -1,6 +1,7 @@
 CC ?= cc
-# Host tests target POSIX systems (glibc under -std=c11 hides fileno/strdup/etc).
-CFLAGS += -std=c11 -D_POSIX_C_SOURCE=200809L -Wall -Wextra -Werror -DHEALTH_TESTABLE -I../orbisrpc -I../third_party/mbedtls/include -I$(SQLITE_DIR)
+# Host tests target POSIX systems (glibc under -std=c11 hides fileno/strdup/etc;
+# _DEFAULT_SOURCE re-enables usleep and friends).
+CFLAGS += -std=c11 -D_POSIX_C_SOURCE=200809L -D_DEFAULT_SOURCE -Wall -Wextra -Werror -DHEALTH_TESTABLE -I../orbisrpc -I../third_party/mbedtls/include -I$(SQLITE_DIR)
 
 TARGET := test_utils
 OBJDIR := .objs