5711dc31d8
Adding `-Wstrict-prototypes` to the default `CFLAGS` in PR #2473 had the non-obvious side-effect of breaking configure-time feature detection, because the [test-program `autoconf` generates for feature detection](https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Generating-Sources.html#:~:text=main%20()) defines `main` as: ```c int main() ``` Which causes all feature checks to fail, since this triggers `-Wstrict-prototypes` and the feature checks use `-Werror`. Resolved by only adding `-Wstrict-prototypes` to `EXTRA_{CFLAGS,CXXFLAGS}` in CI, since these flags are not used during feature detection and we control which compiler is used.