New review comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/36395#discussion_r838667223

Comment:
Appending to variables like `CFLAGS` at the top level is not a good idea because the template is sourced multiple times and the arguments can be repeated. Just set
```sh
CFLAGS="-fPIC"
```
instead. Better still, disable the `unused-function` error here and save the `pre_check` definition:
```sh
CFLAGS="-fPIC -Wno-error=unused-function"
```