unix/Makefile: Remove explicit addition of -std=c++ flag.

This was added merely for building the C++ user module example, so it's a
better fit to add it in the corresponding micropython.mk.

examples/usercmodule/cppexample/micropython.mk

@@ -6,7 +6,7 @@ SRC_USERMOD_CXX += $(CPPEXAMPLE_MOD_DIR)/example.cpp
 
 # Add our module directory to the include path.
 CFLAGS_USERMOD += -I$(CPPEXAMPLE_MOD_DIR)
-CXXFLAGS_USERMOD += -I$(CPPEXAMPLE_MOD_DIR)
+CXXFLAGS_USERMOD += -I$(CPPEXAMPLE_MOD_DIR) -std=c++11
 
 # We use C++ features so have to link against the standard library.
 LDFLAGS_USERMOD += -lstdc++