Don't redefine htonl and ntohl on big-endian

Don't redefine htonl and ntohl on big-endian

Since commit 0fcabdeb52b79775173d009ccc179db104dfbb66, compat/bswap.h
redefined htonl and ntohl to bswap32 not only if bswap32 has been
defined earlier in compat/bswap.h (which is done only on selected
platforms), but also if bswap32 has been defined anywhere else.  This
broke Git at least for NetBSD systems running on big-endian machines
(where ntohl and htonl should, of course, be NOOPs), since NetBSD
defines a bswap32 macro in the system headers.

So, we now undefine any previously defined bswap32 in compat/bswap.h
before defining our own.

Signed-off-by: Holger Weiß <holger@zedat.fu-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>