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AC_PROG_CC_C99 is obsolete starting from autoconf 2.70
## Summary Starting from autoconf 2.70, the macro `AC_PROG_CC_C99` is deprecated (search for `AC_PROG_CC_C99` in the 2.70 [deprecation list](https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/html_node/Obsolete-Macros.html)), so in the future issues like #19655 may happen again. It's probably a good idea to migrate to the newer [`AC_PROG_CC`](https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/html_node/C-Compiler.html#AC_005fPROG_005fCC) macro. If just straight up replacing it is not possible, then maybe consider conditionally invoking the newer version based on the `autoconf` version (like [here](https://gitlab.com/openconnect/openconnect/-/issues/413))? ## Steps to reproduce With an `autoconf` version greater than or equal to 2.70, run `./boot` in any version after #19655 is closed. ## Expected behavior No deprecation warnings should be output. ## Environment * GHC version used: building 9.0.2 with 8.10.7 Optional: * Operating System: Solus 4.3 * System Architecture: Linux x86_64 ## Notes 1. This is definitely not an urgent issue, but would be pretty nice to have. I'd love to fix this if accepted. 2. Maybe take this time to migrate from other deprecated macros too, like `AC_HELP_STRING` and `AC_FOREACH`?
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