Cache detected CPU flags

Originally committed as revision 25085 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/cpu.c

@@ -21,10 +21,17 @@
 
 int av_get_cpu_flags(void)
 {
-    if (ARCH_ARM) return ff_get_cpu_flags_arm();
-    if (ARCH_PPC) return ff_get_cpu_flags_ppc();
-    if (ARCH_X86) return ff_get_cpu_flags_x86();
-    return 0;
+    static int flags, checked;
+
+    if (checked)
+        return flags;
+
+    if (ARCH_ARM) flags = ff_get_cpu_flags_arm();
+    if (ARCH_PPC) flags = ff_get_cpu_flags_ppc();
+    if (ARCH_X86) flags = ff_get_cpu_flags_x86();
+
+    checked = 1;
+    return flags;
 }
 
 #ifdef TEST