Provide strdup() compat

Make strdup() available on all platforms.

app/src/compat.c

@@ -0,0 +1,14 @@
+#include "compat.h"
+
+#include "config.h"
+
+#ifndef HAVE_STRDUP
+char *strdup(const char *s) {
+    size_t size = strlen(s) + 1;
+    char *dup = malloc(size);
+    if (dup) {
+        memcpy(dup, s, size);
+    }
+    return dup;
+}
+#endif