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.qttest.include

@@ -6,6 +6,7 @@ bin_PROGRAMS += qt/test/test_bitcoin-qt
 TESTS += qt/test/test_bitcoin-qt
 
 TEST_QT_MOC_CPP = \
+  qt/test/moc_compattests.cpp \
   qt/test/moc_rpcnestedtests.cpp \
   qt/test/moc_uritests.cpp
 
@@ -14,6 +15,7 @@ TEST_QT_MOC_CPP += qt/test/moc_paymentservertests.cpp
 endif
 
 TEST_QT_H = \
+  qt/test/compattests.h \
   qt/test/rpcnestedtests.h \
   qt/test/uritests.h \
   qt/test/paymentrequestdata.h \
@@ -23,6 +25,7 @@ qt_test_test_bitcoin_qt_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BITCOIN_
   $(QT_INCLUDES) $(QT_TEST_INCLUDES) $(PROTOBUF_CFLAGS)
 
 qt_test_test_bitcoin_qt_SOURCES = \
+  qt/test/compattests.cpp \
   qt/test/rpcnestedtests.cpp \
   qt/test/test_main.cpp \
   qt/test/uritests.cpp \