add b64_ntop checking and fallback for nc(1)

include/compat/resolv.h

@@ -0,0 +1,24 @@
+/*
+ * Public domain
+ * resolv.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_RESOLV_H
+#define LIBCRYPTOCOMPAT_RESOLV_H
+
+#ifdef _MSC_VER
+#if _MSC_VER >= 1900
+#include <../ucrt/resolv.h>
+#else
+#include <../include/resolv.h>
+#endif
+#else
+#include_next <resolv.h>
+#endif
+
+#ifndef HAVE_B64_NTOP
+int b64_ntop(unsigned char const *, size_t, char *, size_t);
+int b64_pton(char const *, unsigned char *, size_t);
+#endif
+
+#endif