diff --git a/src/compat/endian.h b/src/compat/endian.h
--- a/src/compat/endian.h
+++ b/src/compat/endian.h
@@ -19,6 +19,51 @@
 #include <sys/endian.h>
 #endif
 
+#ifndef HAVE_CONFIG_H
+// While not technically a supported configuration, defaulting to defining these
+// DECLs when we were compiled without autotools makes it easier for other build
+// systems to build things like libbitcoinconsensus for strange targets.
+#ifdef htobe16
+#define HAVE_DECL_HTOBE16 1
+#endif
+#ifdef htole16
+#define HAVE_DECL_HTOLE16 1
+#endif
+#ifdef be16toh
+#define HAVE_DECL_BE16TOH 1
+#endif
+#ifdef le16toh
+#define HAVE_DECL_LE16TOH 1
+#endif
+
+#ifdef htobe32
+#define HAVE_DECL_HTOBE32 1
+#endif
+#ifdef htole32
+#define HAVE_DECL_HTOLE32 1
+#endif
+#ifdef be32toh
+#define HAVE_DECL_BE32TOH 1
+#endif
+#ifdef le32toh
+#define HAVE_DECL_LE32TOH 1
+#endif
+
+#ifdef htobe64
+#define HAVE_DECL_HTOBE64 1
+#endif
+#ifdef htole64
+#define HAVE_DECL_HTOLE64 1
+#endif
+#ifdef be64toh
+#define HAVE_DECL_BE64TOH 1
+#endif
+#ifdef le64toh
+#define HAVE_DECL_LE64TOH 1
+#endif
+
+#endif // HAVE_CONFIG_H
+
 #if defined(WORDS_BIGENDIAN)
 
 #if HAVE_DECL_HTOBE16 == 0