Fix dynamic build flags example code

[tfeldmann]

This fixes two problems with the example code:

1. compilation errors like <command-line>:0:9: error: unable to find numeric literal operator 'operator""a5850f' because the git revision macro was not properly escaped
2. The subprocess module returns bytes. In python3 the script would print -DGIT_REV=b'a5850f' because it is not properly decoded. The new code works in python2 and python3.

[CLAassistant]

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[ivankravets]

Thanks for the PR!