Docs support C++11, makefile does not

[Oliver Ford]

Since #4, it is specified that the compilation environment is C99 or C++11.

However, the makefile has not been updated to reflect this.

Suggested fix is to add CXXFLAGS += -std=c++11 (and relatedly, CFLAGS += -std=c99).

[m8pple]

I managed to mess up the commit message, but fixed due to pull. Thanks.