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Update README.md compile instructions to match adding_an_op.
from [adding_an_op](https://github.com/tensorflow/tensorflow/blob/master/tensorflow/docs_src/extend/adding_an_op.md).

Tested: compiled using the modified instructions and ran `python word2vec_optimized_test.py`.

Fixes tensorflow/tensorflow#14620

example_pos_ is the cursor to current position when reading the corpus file. When corpus file is very big, the corpus_size_ may bigger than max of int32, then example_pos_ became negative, and cause `corpus_[example_pos_]` to `Segmentation Fault`.

Remove code and append flag warning

Add case for compiling ops if tensorflow was compiled from source using gcc >= 5.0

Remove the deprecated `scalar_summary` and use `summary.scalar` instead. 

The current program gets the following warning:
WARNING:tensorflow:
build_graph.: scalar_summary (from tensorflow.python.ops.logging_ops) is deprecated and will be removed after 2016-11-30.
Instructions for updating:
Please switch to tf.summary.scalar. Note that tf.summary.scalar uses the node name instead of the tag. This means that TensorFlow will automatically de-duplicate summary names based on the scope they are created in. Also, passing a tensor or list of tags to a scalar summary op is no longer supported.

bug fix for contrib.deprecated eliminatation in tf version 12.