[PATCH] fix altivec.h detection

Patch from Gentoo:

http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/media-libs/libmpeg2/files/libmpeg2-0.5.1-altivec.patch?revision=1.1&view=markup

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
--- a/configure.ac.bak	2010-05-25 17:12:14.756245990 +0000
+++ b/configure.ac	2010-05-25 17:11:51.629581723 +0000
@@ -79,11 +79,10 @@
 		 CFLAGS="$OPT_CFLAGS $TRY_CFLAGS $CFLAGS"
 		 AC_MSG_CHECKING([if <altivec.h> is needed])
 		 AC_TRY_COMPILE([],
-		    [typedef vector int t;
-		     vec_ld(0, (unsigned char *)0);],
+		    [vector int t; t = vec_add(t,t);],
 		    [have_altivec=yes; AC_MSG_RESULT(no)],
 		    [AC_TRY_COMPILE([#include <altivec.h>],
-			[typedef vector int t; vec_ld(0, (unsigned char *)0);],
+			[vector int t; t = vec_add(t,t);],
 			[AC_DEFINE([HAVE_ALTIVEC_H],,
 			    [Define to 1 if you have the <altivec.h> header.])
 			 have_altivec=yes; AC_MSG_RESULT(yes)],