New review comment by Duncaen on void-packages repository

https://github.com/void-linux/void-packages/pull/33208#discussion_r720461409

Comment:
We can't set it at the toplevel because the I think xbps-src adds its own CFLAGS etc, we theoretically don't need it in do_build, but it makes working on the build a whole lot easier since ninja might need to rebuild some ninja files.