Add clang to detected compilers

Originally committed as revision 20350 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1590,6 +1590,11 @@ elif $cc -version 2>/dev/null | grep -q TMS470; then
             esac
         done
     }
+elif $cc -v 2>&1 | grep -q clang; then
+    cc_type=clang
+    cc_version=__VERSION__
+    CC_DEPFLAGS='-MMD'
+    AS_DEPFLAGS='-MMD'
 fi
 
 test -n "$cc_type" && enable $cc_type || echolog "Unknown C compiler $cc"
@@ -1615,6 +1620,10 @@ if test -n "$sysroot"; then
             add_cppflags -I"$sysinclude"
             add_ldflags  --sysroot="$sysroot"
         ;;
+        clang)
+            add_cppflags -isysroot="$sysroot"
+            add_ldflags -isysroot="$sysroot"
+        ;;
     esac
 fi
 
@@ -2464,6 +2473,8 @@ elif enabled ccc; then
     add_cflags -msg_disable unreachcode
 elif enabled gcc; then
     check_cflags -fno-tree-vectorize
+elif enabled clang; then
+    check_cflags -Qunused-arguments
 fi
 
 if enabled gprof; then