Remove custom GCC warn flags

To fix build error on macOS where g++ is actually clang++:

    error: unknown warning option '-Wstrict-null-sentinel' [-Werror,-Wunknown-warning-option]

See also:

- https://travis-ci.org/weisslj/cpp-optparse/jobs/292772674
- https://stackoverflow.com/questions/11714827/how-to-turn-on-literally-all-of-gccs-warnings
- https://stackoverflow.com/questions/19535422/os-x-10-9-gcc-links-to-clang
- https://docs.travis-ci.com/user/languages/c/#GCC-on-OS-X

Author: weisslj

Makefile

@@ -1,6 +1,6 @@
 ifeq ($(WARN),1)
 ifeq ($(CXX),g++)
-WARN_FLAGS = -O3 -g -Wall -Wextra -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Wstrict-null-sentinel -Woverloaded-virtual -Wshadow -Wcast-align -Wpointer-arith -Wwrite-strings -Wundef -Wredundant-decls -Werror # -Weffc++
+WARN_FLAGS = -O3 -g -Wall -Wextra -Werror # -Weffc++
 else ifeq ($(CXX),clang++)
 WARN_FLAGS = -O3 -g -Werror
 else ifeq ($(CXX),icpc)