Don't redefine htonl and ntohl on big-endian

Since commit 0fcabdeb52, 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>

compat/bswap.h

@@ -17,6 +17,8 @@ static inline uint32_t default_swab32(uint32_t val)
 		((val & 0x000000ff) << 24));
 }
 
+#undef bswap32
+
 #if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
 
 #define bswap32(x) ({ \