Use <byteswap.h> if present.

Patches applied:

 * erikd@miles--2004/libsndfile--stable--0--patch-3
   ChangeLog

ChangeLog

@@ -1,3 +1,20 @@
+2004-07-17  Erik de Castro Lopo  <erikd AT mega-nerd DOT com>
+
+    * src/pcm.c
+    When converting from unsigned char to float or double, subtract 128 before
+    converting to float/double rather than after to save a floating point
+    operation as suggested by Stefan Briesenick.
+
+    * src/(pcm|sfendian|alaw|ulaw|double64|float32).c
+    Optimize inner loops by changing the loop counting slightly as suggested
+    by Stefan Briesenick.
+
+    * configure.ac
+    Detect presence of <byteswap.h>.
+
+    * src/sfendian.h
+    Use <byteswap.h> if present.
+
 2004-07-02  Erik de Castro Lopo  <erikd AT mega-nerd DOT com>
 
     * src/common.h src/*.c

configure.ac

@@ -44,6 +44,7 @@ if test "x-$build_os-$host_os-$target_os" = x-mingw32-mingw32-mingw32 ; then
 AC_HEADER_STDC
 
 AC_CHECK_HEADERS(endian.h)
+AC_CHECK_HEADERS(byteswap.h)
 AC_HEADER_SYS_WAIT
 
 AC_CHECK_DECLS(S_IRGRP)

src/sfendian.h

@@ -16,7 +16,21 @@
 ** Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */
 
-#include	"config.h"
+#include "config.h"
+
+#if HAVE_BYTESWAP_H
+
+#include <byteswap.h>
+
+#define	ENDSWAP_SHORT(x)	bswap_16 (x)
+#define	ENDSWAP_INT(x)		bswap_32 (x)
+
+#else
+
+#define	ENDSWAP_SHORT(x)	((((x)>>8)&0xFF)+(((x)&0xFF)<<8))
+#define	ENDSWAP_INT(x)		((((x)>>24)&0xFF)+(((x)>>8)&0xFF00)+(((x)&0xFF00)<<8)+(((x)&0xFF)<<24))
+
+#endif
 
 /*
 ** Many file types (ie WAV, AIFF) use sets of four consecutive bytes as a
@@ -33,10 +47,6 @@
 	#error "Target CPU endian-ness unknown. May need to hand edit src/config.h"
 #endif
 
-/* wo standard endswap macros. */
-
-#define	ENDSWAP_SHORT(x)	((((x)>>8)&0xFF)+(((x)&0xFF)<<8))
-#define	ENDSWAP_INT(x)		((((x)>>24)&0xFF)+(((x)>>8)&0xFF00)+(((x)&0xFF00)<<8)+(((x)&0xFF)<<24))
 /*
 ** Macros to handle reading of data of a specific endian-ness into host endian
 ** shorts and ints. The single input is an unsigned char* pointer to the start