Rework byteswap.h inclusion.

This header is not available on e.g. FreeBSD so detect it's presence in configure.ac and only include it if appropriate.  Then, if byte order manipulation macros are not defined (old glibc like the one provided by Debian GNU/Linux) define them.

configure.ac

@@ -33,6 +33,8 @@ if test $ac_cv_header_endian_h = "no" -a $ac_cv_header_sys_endian_h = "no"; then
   AC_MSG_ERROR(["Can't locate header endian.h."]);
 fi
 
+AC_CHECK_HEADERS([byteswap.h])
+
 AC_DEFINE([_XOPEN_SOURCE], [600], [Define to 500 if Single Unix conformance is wanted, 600 for sixth revision.])
 
 CFLAGS="$CFLAGS -std=c99"

libfreefare/freefare_internal.h

@@ -20,8 +20,11 @@
 #ifndef __FREEFARE_INTERNAL_H__
 #define __FREEFARE_INTERNAL_H__
 
+#include "config.h"
+
+#if defined(HAVE_BYTESWAP_H)
+#include <byteswap.h>
 #if !defined(le32toh) || !defined(htole32)
-  #include <byteswap.h>
   #if BYTE_ORDER == LITTLE_ENDIAN
     #define le32toh(x) (x)
     #define htole32(x) bswap_32(x)
@@ -30,6 +33,7 @@
     #define htole32(x) (x)
   #endif
 #endif
+#endif
 
 
 struct mad_sector_0x00;