372f03ba20
glibc 2.30 and up emit an ugly depreciation warning on `#include <sys/sysctl.h>` - this patch makes the build system fail the include test for `sys/sysctl.h` by forcibly setting `-Werror` before the call to `check_include_files` (which internally uses `try_compile`) to get `HAVE_SYS_SYSCTL` to not be defined (even if it's there) if it would cause such a depreciation message to be emitted. Ideally, we would not have to manually massage `CMAKE_C_FLAGS` before calling `check_include_files` and could just tweak that to either always or conditionally try compilation with `-Werror`, but try_compile doesn't actually use any overridden `CMAKE_C_FLAGS` values [0] (dating back to 2006). [0]: https://cmake.org/pipermail/cmake/2006-October/011649.html