There's a closed pull request on the void-packages repository

gcc + cross: remove default definition of __gnu_linux__ on ppc*-musl
https://github.com/void-linux/void-packages/pull/11524
Description: The ppc compilers on musl define the `__gnu_linux__` macro by default, while on other architectures they do not. 

Expected output:
```
$ gcc -dM -E - < /dev/null|grep gnu_linux
$
```
Output before patching:
```
$ gcc -dM -E - < /dev/null|grep gnu_linux
#define __gnu_linux__ 1
$
```

This breaks compilation of gimp and maybe other things. It is fixed in gcc 9.1, but the patch is a part of a bigger refactor that's too big to apply, so just do it conditionally like this, we can remove it once we update gcc.