Turn on -Wall -Wextra for clang++

TODO: -Weverything

See also:

- https://stackoverflow.com/questions/24904101/what-warnings-are-included-in-clangs-wall-and-wextra
- http://amattn.com/p/better_apps_clang_weverything_or_wall_is_a_lie.html

Makefile

@@ -2,7 +2,7 @@ ifeq ($(WARN),1)
 ifeq ($(CXX),g++)
 WARN_FLAGS = -O3 -g -Wall -Wextra -Werror # -Weffc++
 else ifeq ($(CXX),clang++)
-WARN_FLAGS = -O3 -g -Werror
+WARN_FLAGS = -O3 -g -Wall -Wextra -Werror
 else ifeq ($(CXX),icpc)
 WARN_FLAGS = -O3 -ipo -g -Wall -wd981 -wd383 -wd2259 -Werror # -Weffc++
 endif