Uses built-in byte swap if available (Apple) and if bswap_XX is undefined.

Defers to pre-defined version if found (e.g. protobuf). For protobuf case, the definitions are identical and thus include order should not affect results.

src/Makefile.test.include

@@ -52,6 +52,7 @@ BITCOIN_TESTS =\
   test/bip32_tests.cpp \
   test/blockencodings_tests.cpp \
   test/bloom_tests.cpp \
+  test/bswap_tests.cpp \
   test/coins_tests.cpp \
   test/compress_tests.cpp \
   test/crypto_tests.cpp \