configure: Add usan to the toolchain presets

clang-3.4 and gcc-4.9 have it.

configure

@@ -2349,6 +2349,11 @@ case "$toolchain" in
         add_cflags  -fsanitize=thread -pie
         add_ldflags -fsanitize=thread -pie
     ;;
+    clang-usan)
+        cc_default="clang"
+        add_cflags  -fsanitize=undefined
+        add_ldflags -fsanitize=undefined
+    ;;
     gcc-asan)
         cc_default="gcc"
         add_cflags  -fsanitize=address
@@ -2359,6 +2364,11 @@ case "$toolchain" in
         add_cflags  -fsanitize=thread -pie -fPIC
         add_ldflags -fsanitize=thread -pie -fPIC
     ;;
+    gcc-usan)
+        cc_default="gcc"
+        add_cflags  -fsanitize=undefined
+        add_ldflags -fsanitize=undefined
+    ;;
     valgrind-massif)
         target_exec_default="valgrind"
         target_exec_args="--alloc-fn=av_malloc --alloc-fn=av_mallocz"