Fix build failure on osx 10.5.8 ppc

Second parameter to vec_splat must be a literal, not a variable
value.  Therefore the second nested for-loop in
float_to_int16_stride_altivec had to be unrolled.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ppc/fmtconvert_altivec.c

@@ -86,16 +86,27 @@ static void float_to_int16_altivec(int16_t *dst, const float *src, long len)
 static void float_to_int16_stride_altivec(int16_t *dst, const float *src,
                                           long len, int stride)
 {
-    int i, j;
+    int i;
     vector signed short d, s;
 
     for (i = 0; i < len - 7; i += 8) {
         d = float_to_int16_one_altivec(src + i);
-        for (j = 0; j < 8; j++) {
-            s = vec_splat(d, j);
-            vec_ste(s, 0, dst);
-            dst += stride;
-        }
+
+#define ASSIGN_S_VEC_SPLAT_D(j) \
+        s = vec_splat(d, j); \
+        vec_ste(s, 0, dst); \
+        dst += stride
+
+        ASSIGN_S_VEC_SPLAT_D(0);
+        ASSIGN_S_VEC_SPLAT_D(1);
+        ASSIGN_S_VEC_SPLAT_D(2);
+        ASSIGN_S_VEC_SPLAT_D(3);
+        ASSIGN_S_VEC_SPLAT_D(4);
+        ASSIGN_S_VEC_SPLAT_D(5);
+        ASSIGN_S_VEC_SPLAT_D(6);
+        ASSIGN_S_VEC_SPLAT_D(7);
+
+#undef ASSIGN_S_VEC_SPLAT_D
     }
 }