Here we have endian.h but also need some other compat shims; the build was failing with an unclear "can't include machine/endian.h". Alter our local endian.h to #include_next endian.h. Then, move the various #define HAVE_* before including any header in config.h since some of the included headers were pulling in endian.h before HAVE_ENDIAN_H is defined.

As usual, if not found try sys/endian.h or pull in the macos thingy.