configure: clean up Altivec detection

There used to be one test for Altivec intrinsics support and a
separate test to determine which of two possible syntaxes to use
for vector literals.  Since 2008, we only support the more common
of these so the split test no longer makes sense.

This combines the tests into one and also changes the hard error on
failure to a warning.  The test can reasonably fail if no --cpu flag
is provided (or is provided with an unknown CPU) and the compiler
default target does not support Altivec.  Aborting in this case is
probably over-reacting.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2875,17 +2875,14 @@ elif enabled ppc; then
         check_cc <<EOF || disable altivec
 $inc_altivec_h
 int main(void) {
-    vector signed int v1, v2, v3;
-    v1 = vec_add(v2,v3);
+    vector signed int v1 = (vector signed int) { 0 };
+    vector signed int v2 = (vector signed int) { 1 };
+    v1 = vec_add(v1, v2);
     return 0;
 }
 EOF
 
-        # check if our compiler supports braces for vector declarations
-        check_cc <<EOF || die "You need a compiler that supports {} in AltiVec vector declarations."
-$inc_altivec_h
-int main (void) { (vector int) {1}; return 0; }
-EOF
+        enabled altivec || warn "Altivec disabled, possibly missing --cpu flag"
     fi
 
 elif enabled sparc; then