lsws/ppc/yuv2rgb_altivec: Fix build in non-VSX environments with Clang

Add a check for the existence of the vec_xl() function. Clang provides
the function even with VSX not enabled.

configure

@@ -2224,6 +2224,7 @@ ARCH_EXT_LIST_PPC="
     ldbrx
     power8
     ppc4xx
+    vec_xl
     vsx
 "
 
@@ -2777,6 +2778,7 @@ altivec_deps="ppc"
 dcbzl_deps="ppc"
 ldbrx_deps="ppc"
 ppc4xx_deps="ppc"
+vec_xl_deps="altivec"
 vsx_deps="altivec"
 power8_deps="vsx"
 
@@ -6421,6 +6423,11 @@ elif enabled ppc; then
         check_cpp_condition power8 "altivec.h" "defined(_ARCH_PWR8)"
     fi
 
+    if enabled altivec && disabled vsx; then
+        check_cc vec_xl altivec.h "const unsigned char *y1i = { 0 };
+                                   vector unsigned char y0 = vec_xl(0, y1i);"
+    fi
+
 elif enabled riscv; then
 
     enabled rv && check_inline_asm rv '".option arch, +zbb\nrev8 t0, t1"'
@@ -7991,6 +7998,7 @@ if enabled ppc; then
     echo "POWER8 enabled            ${power8-no}"
     echo "PPC 4xx optimizations     ${ppc4xx-no}"
     echo "dcbzl available           ${dcbzl-no}"
+    echo "vec_xl available          ${vec_xl-no}"
 fi
 if enabled loongarch; then
     echo "LSX enabled               ${lsx-no}"

libswscale/ppc/yuv2rgb_altivec.c

@@ -284,7 +284,7 @@ static inline void cvtyuvtoRGB(SwsInternal *c, vector signed short Y,
  * ------------------------------------------------------------------------------
  */
 
-#if !HAVE_VSX
+#if !HAVE_VEC_XL
 static inline vector unsigned char vec_xl(signed long long offset, const ubyte *addr)
 {
     const vector unsigned char *v_addr = (const vector unsigned char *) (addr + offset);
@@ -292,7 +292,7 @@ static inline vector unsigned char vec_xl(signed long long offset, const ubyte *
 
     return (vector unsigned char) vec_perm(v_addr[0], v_addr[1], align_perm);
 }
-#endif /* !HAVE_VSX */
+#endif /* !HAVE_VEC_XL */
 
 #define DEFCSP420_CVT(name, out_pixels)                                       \
 static int altivec_ ## name(SwsInternal *c, const unsigned char *const *in,   \