build: Endian compatibility

- Detect endian instead of stopping configure on big-endian
- Add `byteswap.h` and `endian.h` header for compatibility with
  Windows and other operating systems that don't come with them
- Update `crypto/common.h` functions to use compat
  endian header

src/compat/byteswap.h

@@ -0,0 +1,47 @@
+// Copyright (c) 2014 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_COMPAT_BYTESWAP_H
+#define BITCOIN_COMPAT_BYTESWAP_H
+
+#if defined(HAVE_CONFIG_H)
+#include "config/bitcoin-config.h"
+#endif
+
+#include <stdint.h>
+
+#if defined(HAVE_BYTESWAP_H)
+#include <byteswap.h>
+#endif
+
+#if HAVE_DECL_BSWAP_16 == 0
+inline uint16_t bswap_16(uint16_t x)
+{
+    return (x >> 8) | ((x & 0x00ff) << 8);
+}
+#endif // HAVE_DECL_BSWAP16
+
+#if HAVE_DECL_BSWAP_32 == 0
+inline uint32_t bswap_32(uint32_t x)
+{
+    return (((x & 0xff000000U) >> 24) | ((x & 0x00ff0000U) >>  8) |
+            ((x & 0x0000ff00U) <<  8) | ((x & 0x000000ffU) << 24));
+}
+#endif // HAVE_DECL_BSWAP32
+
+#if HAVE_DECL_BSWAP_64 == 0
+inline uint64_t bswap_64(uint64_t x)
+{
+     return (((x & 0xff00000000000000ull) >> 56)
+          | ((x & 0x00ff000000000000ull) >> 40)
+          | ((x & 0x0000ff0000000000ull) >> 24)
+          | ((x & 0x000000ff00000000ull) >> 8)
+          | ((x & 0x00000000ff000000ull) << 8)
+          | ((x & 0x0000000000ff0000ull) << 24)
+          | ((x & 0x000000000000ff00ull) << 40)
+          | ((x & 0x00000000000000ffull) << 56));
+}
+#endif // HAVE_DECL_BSWAP64
+
+#endif // BITCOIN_COMPAT_BYTESWAP_H

src/compat/endian.h

@@ -0,0 +1,194 @@
+// Copyright (c) 2014 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_COMPAT_ENDIAN_H
+#define BITCOIN_COMPAT_ENDIAN_H
+
+#if defined(HAVE_CONFIG_H)
+#include "config/bitcoin-config.h"
+#endif
+
+#include <stdint.h>
+
+#include "compat/byteswap.h"
+
+#if defined(HAVE_ENDIAN_H)
+#include <endian.h>
+#endif
+
+#if defined(WORDS_BIGENDIAN)
+
+#if HAVE_DECL_HTOBE16 == 0
+inline uint16_t htobe16(uint16_t host_16bits)
+{
+    return host_16bits;
+}
+#endif // HAVE_DECL_HTOBE16
+
+#if HAVE_DECL_HTOLE16 == 0
+inline uint16_t htole16(uint16_t host_16bits)
+{
+    return bswap_16(host_16bits);
+}
+#endif // HAVE_DECL_HTOLE16
+
+#if HAVE_DECL_BE16TOH == 0
+inline uint16_t be16toh(uint16_t big_endian_16bits)
+{
+    return big_endian_16bits;
+}
+#endif // HAVE_DECL_BE16TOH
+
+#if HAVE_DECL_LE16TOH == 0
+inline uint16_t le16toh(uint16_t little_endian_16bits)
+{
+    return bswap_16(little_endian_16bits);
+}
+#endif // HAVE_DECL_LE16TOH
+
+#if HAVE_DECL_HTOBE32 == 0
+inline uint32_t htobe32(uint32_t host_32bits)
+{
+    return host_32bits;
+}
+#endif // HAVE_DECL_HTOBE32
+
+#if HAVE_DECL_HTOLE32 == 0
+inline uint32_t htole32(uint32_t host_32bits)
+{
+    return bswap_32(host_32bits);
+}
+#endif // HAVE_DECL_HTOLE32
+
+#if HAVE_DECL_BE32TOH == 0
+inline uint32_t be32toh(uint32_t big_endian_32bits)
+{
+    return big_endian_32bits;
+}
+#endif // HAVE_DECL_BE32TOH
+
+#if HAVE_DECL_LE32TOH == 0
+inline uint32_t le32toh(uint32_t little_endian_32bits)
+{
+    return bswap_32(little_endian_32bits);
+}
+#endif // HAVE_DECL_LE32TOH
+
+#if HAVE_DECL_HTOBE64 == 0
+inline uint64_t htobe64(uint64_t host_64bits)
+{
+    return host_64bits;
+}
+#endif // HAVE_DECL_HTOBE64
+
+#if HAVE_DECL_HTOLE64 == 0
+inline uint64_t htole64(uint64_t host_64bits)
+{
+    return bswap_64(host_64bits);
+}
+#endif // HAVE_DECL_HTOLE64
+
+#if HAVE_DECL_BE64TOH == 0
+inline uint64_t be64toh(uint64_t big_endian_64bits)
+{
+    return big_endian_64bits;
+}
+#endif // HAVE_DECL_BE64TOH
+
+#if HAVE_DECL_LE64TOH == 0
+inline uint64_t le64toh(uint64_t little_endian_64bits)
+{
+    return bswap_64(little_endian_64bits);
+}
+#endif // HAVE_DECL_LE64TOH
+
+#else // WORDS_BIGENDIAN
+
+#if HAVE_DECL_HTOBE16 == 0
+inline uint16_t htobe16(uint16_t host_16bits)
+{
+    return bswap_16(host_16bits);
+}
+#endif // HAVE_DECL_HTOBE16
+
+#if HAVE_DECL_HTOLE16 == 0
+inline uint16_t htole16(uint16_t host_16bits)
+{
+    return host_16bits;
+}
+#endif // HAVE_DECL_HTOLE16
+
+#if HAVE_DECL_BE16TOH == 0
+inline uint16_t be16toh(uint16_t big_endian_16bits)
+{
+    return bswap_16(big_endian_16bits);
+}
+#endif // HAVE_DECL_BE16TOH
+
+#if HAVE_DECL_LE16TOH == 0
+inline uint16_t le16toh(uint16_t little_endian_16bits)
+{
+    return little_endian_16bits;
+}
+#endif // HAVE_DECL_LE16TOH
+
+#if HAVE_DECL_HTOBE32 == 0
+inline uint32_t htobe32(uint32_t host_32bits)
+{
+    return bswap_32(host_32bits);
+}
+#endif // HAVE_DECL_HTOBE32
+
+#if HAVE_DECL_HTOLE32 == 0
+inline uint32_t htole32(uint32_t host_32bits)
+{
+    return host_32bits;
+}
+#endif // HAVE_DECL_HTOLE32
+
+#if HAVE_DECL_BE32TOH == 0
+inline uint32_t be32toh(uint32_t big_endian_32bits)
+{
+    return bswap_32(big_endian_32bits);
+}
+#endif // HAVE_DECL_BE32TOH
+
+#if HAVE_DECL_LE32TOH == 0
+inline uint32_t le32toh(uint32_t little_endian_32bits)
+{
+    return little_endian_32bits;
+}
+#endif // HAVE_DECL_LE32TOH
+
+#if HAVE_DECL_HTOBE64 == 0
+inline uint64_t htobe64(uint64_t host_64bits)
+{
+    return bswap_64(host_64bits);
+}
+#endif // HAVE_DECL_HTOBE64
+
+#if HAVE_DECL_HTOLE64 == 0
+inline uint64_t htole64(uint64_t host_64bits)
+{
+    return host_64bits;
+}
+#endif // HAVE_DECL_HTOLE64
+
+#if HAVE_DECL_BE64TOH == 0
+inline uint64_t be64toh(uint64_t big_endian_64bits)
+{
+    return bswap_64(big_endian_64bits);
+}
+#endif // HAVE_DECL_BE64TOH
+
+#if HAVE_DECL_LE64TOH == 0
+inline uint64_t le64toh(uint64_t little_endian_64bits)
+{
+    return little_endian_64bits;
+}
+#endif // HAVE_DECL_LE64TOH
+
+#endif // WORDS_BIGENDIAN
+
+#endif // BITCOIN_COMPAT_ENDIAN_H