[evmar]

The code:

    //a decent amount of this was copied/modified from backward.cpp (https://github.com/bombela/backward-cpp)

The license on the other side of that link:

    The above copyright notice and this permission notice shall be included in all
    copies or substantial portions of the Software.

[Rexxar]

Both have "MIT License" and he explicitly acknowledge the source. He probably forgot that he had to add an additional line with "Copyright 2013 Google Inc." in his own license file.

[londons_explore]

This is only an issue if the original author chooses to enforce. You don't know - this code may have been given special permission to omit the notice by the original author. It isn't up to random joe to find copyrights they think have been violated.