libavutil/ppc/cpu.c: check that AT_HWCAP2 is defined

It was not introduced until glibc 2.18.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d799ad2404)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/ppc/cpu.c

@@ -95,12 +95,15 @@ int ff_get_cpu_flags_ppc(void)
 #endif
                 if (ret & AV_CPU_FLAG_VSX)
                     av_assert0(ret & AV_CPU_FLAG_ALTIVEC);
-            } else if (buf[i] == AT_HWCAP2) {
+            }
+#ifdef AT_HWCAP2 /* not introduced until glibc 2.18 */
+            else if (buf[i] == AT_HWCAP2) {
 #ifdef PPC_FEATURE2_ARCH_2_07
                 if (buf[i + 1] & PPC_FEATURE2_ARCH_2_07)
                     ret |= AV_CPU_FLAG_POWER8;
 #endif
             }
+#endif /* AT_HWCAP2 */
         }
     }