Default Makefile to C++11

I assert that ~all non-C++11 usage remaining is on Windows, which uses
CMake; therefore, let’s have Makefile default C++11 to enabled.

Also, add a check for LLVM3.6 to also force C++11 on. (Make has no >=
operator, but by the time there’s LLVM3.7 we’ll require C++11
everywhere anyway)