test: Add new overlay unit test

test/.gitignore

@@ -1 +1,2 @@
 headers
+overlay

test/Makefile.am

@@ -12,6 +12,7 @@ AM_CPPFLAGS = \
 
 check_PROGRAMS = \
 	headers \
+	overlay \
 	$(nil)
 
 TESTS = $(check_PROGRAMS)

test/overlay.c

@@ -0,0 +1,23 @@
+/* Include system headers that are “known” to pull bits selectively from
+ * other headers through magic macros, to check that the overlay is working
+ * properly. */
+#include <errno.h>
+#include <grp.h>
+#include <stdint.h>
+
+/* Include libbsd overlayed headers that might get partially included. */
+#include <sys/cdefs.h>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+int
+main()
+{
+	/* Test that we do not get partial definitions. */
+	fflush(stdout);
+
+	return 0;
+}