Dummy issetugid for Linux.

configure

@@ -230,6 +230,12 @@ main() {
 }
 EOF
 
+test_compile issetugid issetugid.o <<EOF
+main() {
+	int i = issetugid();
+}
+EOF
+
 test_compile setresgid setresgid.o <<EOF
 main() {
 	int i = setresgid(9999, 9999, 9999);

src/issetugid.c

@@ -0,0 +1,13 @@
+/* Copyright (c) 2019 Bertrand Janin <[email protected]> */
+
+/*
+ * XXX: This is not an implementation of issetugid but since this call is made
+ * to check on how to use pledge() and pledge is not available on any of the
+ * systems we target, we can safely use a dummy version.
+ */
+
+int
+issetugid(void)
+{
+	return 0;
+}