Fix compilation for linux-ppc

Check for HAVE_BYTESWAP_H was missing from configure.  Also have_altivec() is
no longer in ffmpeg, so I reimplemented it using av_get_cpu_flags().

mythtv/configure

@@ -1229,6 +1229,7 @@ HAVE_LIST="
     attribute_may_alias
     attribute_packed
     bswap
+    byteswap_h
     closesocket
     cmov
     conio_h

mythtv/libs/libmythtv/yuv2rgb.cpp

@@ -40,7 +40,10 @@ extern "C" {
 #endif
 
 #if HAVE_ALTIVEC
-extern "C" int has_altivec(void);    // in libavcodec/ppc/check_altivec.c
+extern "C" {
+#include "libavutil/cpu.h"
+}
+int has_altivec(void); 
 #if HAVE_ALTIVEC_H
 #include <altivec.h>
 #else
@@ -49,6 +52,17 @@ extern "C" int has_altivec(void);    // in libavcodec/ppc/check_altivec.c
 #endif
 #include "yuv2rgb.h"
 
+#if HAVE_ALTIVEC
+int has_altivec(void)
+{
+    int cpu_flags = av_get_cpu_flags();
+    if (cpu_flags & AV_CPU_FLAG_ALTIVEC)
+        return(1);
+
+    return(0);
+}
+#endif
+
 /** \file yuv2rgb.cpp
  *  \brief Contains various YUV, VUY and RGBA colorspace conversion routines.
  *