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'.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@12445 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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>" ||