add -std=c++11

[jli05]

Add option -std=c++11 otherwise guide/ won't make.

In order to make this pass Travis tests, I had to change the default C++ compiler to clang++ so that it'd recognise the latest compilation options. The g++ bundled in the Travis container is of an old version circa 3.7.

In guide/Makefile we won't export again CXX if it's already defined.

[szha]

This code base has been donated to the Apache MXNet project per #373, and repo is deprecated. Future development should continue in Apache MXNet.

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