diff --git a/test/Makefile b/test/Makefile
index 9dcc52336..ed8c34bf8 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -28,6 +28,7 @@ fault-injection: .FORCE
 zdtm_ns: $(shell echo "$(TST)" | tr ' ' '\n' | grep -P $(EXP))
 zdtm_nons: $(shell echo "$(TST)" | tr ' ' '\n' | grep -vP $(EXP))
 
+override CFLAGS += -D_GNU_SOURCE
 zdtm_ct: zdtm_ct.c
 
 clean:
diff --git a/test/zdtm_ct.c b/test/zdtm_ct.c
index 1374f786f..3471655fd 100644
--- a/test/zdtm_ct.c
+++ b/test/zdtm_ct.c
@@ -1,10 +1,10 @@
-#define _GNU_SOURCE
 #include <sched.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <sys/mount.h>
+#include <unistd.h>
 
 int main(int argc, char **argv)
 {