This is an auto-filed bug because sci-electronics/magic calls cpp directly.
The issue was originally discovered on amd64, but it may be reproducible on other arches as well.
If you think that a different summary clarifies the issue better, feel free to change it.
Attached build log and emerge --info.
If you think it doesn't make sense fix these type of issues, I'd like to point out that won't be possible use a different CPP implementation (like clang-cpp) by setting the CPP variable. So this issue has been reproduced by setting the CPP variable to x86_64-pc-linux-gnu-gcc -E and by removing the /usr/bin/cpp binary.
Created attachment 641820 [details]
build log and emerge --info
Fixed in 8.3.232 (commit 6f98b5322329ec776ad9f3ec153a6f9490f82772).