Set HAVE_FAST_CLZ if CPU has a fast count leading zeros instruction

Originally committed as revision 21216 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -962,6 +962,7 @@ HAVE_LIST="
     exp2
     exp2f
     fast_64bit
+    fast_clz
     fast_cmov
     fast_unaligned
     fork
@@ -1094,6 +1095,8 @@ sse_deps="mmx"
 ssse3_deps="sse"
 vis_deps="sparc"
 
+fast_clz_if_any="alpha armv5te avr32 mips ppc x86"
+
 need_memalign="altivec neon sse"
 inline_asm_deps="!tms470"
 
@@ -2312,6 +2315,12 @@ EOF
     esac
     disabled yasm || { check_yasm "pabsw xmm0, xmm0" && enable yasm; }
 
+    case "$cpu" in
+        athlon*|opteron*|k8*|pentium|pentium-mmx|prescott|nocona|atom|geode)
+            disable fast_clz
+        ;;
+    esac
+
 fi
 
 if check_func dlopen; then