Do not set COMPILE_C if AltiVec is enabled without runtime CPU detection.

Gets rid of the following warning:
postprocess_template.c:3271: warning: ‘postProcess_C’ defined but not used

Originally committed as revision 16055 to svn://svn.ffmpeg.org/ffmpeg/trunk

libpostproc/postprocess.c

@@ -557,7 +557,7 @@ static av_always_inline void do_a_deblock_C(uint8_t *src, int step, int stride,
 
 //Note: we have C, MMX, MMX2, 3DNOW version there is no 3DNOW+MMX2 one
 //Plain C versions
-#if !defined (HAVE_MMX) || defined (RUNTIME_CPUDETECT)
+#if !(defined (HAVE_MMX) || defined (HAVE_ALTIVEC)) || defined (RUNTIME_CPUDETECT)
 #define COMPILE_C
 #endif