handle both endianness macros defined

Author: grisumbras

include/boost/json/detail/config.hpp

@@ -260,6 +260,8 @@
 # else
 #  error The Boost.JSON library could not determine the endianness of this platform. Define either BOOST_JSON_BIG_ENDIAN or BOOST_JSON_LITTLE_ENDIAN.
 # endif
+#elif defined(BOOST_JSON_BIG_ENDIAN) && defined(BOOST_JSON_LITTLE_ENDIAN)
+# error Both BOOST_JSON_BIG_ENDIAN and BOOST_JSON_LITTLE_ENDIAN are defined. You should define only one of them.
 #endif
 
 #if defined(__cpp_constinit) && __cpp_constinit >= 201907L