Fix default flags

Makefile

@@ -11,9 +11,23 @@ else
 UNAME_S := $(shell uname -s)
 endif
 
+# For Darwin (Mac OS X / macOS) we assume that the default compiler
+# clang++ is used; when $(CXX) is some version of g++, then
+# $(CXXSTD) has to be set to -std=c++11 (or newer) so
+# that -stdlib=libc++ is not automatically added.
+
+ifeq ($(CXXSTD),)
+CXXSTD := -std=c++11
+ifeq ($(UNAME_S),Darwin)
+CXXSTD += -stdlib=libc++
+endif
+endif
+
+# Ensure strict standard compliance and no warnings, can be
+# changed if desired.
+
 CPPFLAGS ?= -pedantic
-CXXSTD ?= c++11
-CXXFLAGS ?= -O3 -Wall -Wextra -Werror
+CXXFLAGS ?= -Wall -Wextra -Wshadow -Werror -O3
 
 CLANG_TIDY ?= clang-tidy