Only check for altivec.h when not using Apple gcc and not adding -faltivec to

CFLAGS. This avoids a couple of warnings of the type
'#warning: Ignoring <altivec.h> because "-faltivec" specified'.

Originally committed as revision 12445 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1536,10 +1536,9 @@ if enabled altivec; then
         add_cflags "-faltivec"
     else
         add_cflags "-maltivec -mabi=altivec"
+        check_header altivec.h
     fi
 
-    check_header altivec.h
-
     # check if our compiler supports Motorola AltiVec C API
     enabled altivec_h &&
         inc_altivec_h="#include <altivec.h>" ||