FLINT and ENV.runtime_cpu_detection

[edgarcosta]

Description of issue

I am struggling to fix this PR: Homebrew/homebrew-core#166490

In particular, ENV.runtime_cpu_detection is necessary. Otherwise the ./configure will generate headers containing blocks like:

#if defined(__AVX2__)
...
#elif defined(__ARM_NEON) || defined(_M_ARM64)
...
#else

#error machine_vector.h not implemented

#endif

which will not compile, as without the flag, we remove arguments like -march=skylake

But the Linux runner complains:

Full audit --formula flint --online --git --skip-style output
  flint
    * No `cpuid` instruction detected. flint should not use `ENV.runtime_cpu_detection`.
  Error: 1 problem in 1 formula detected. 

despite FLINT using CPUID at the configure time to generate the appropriate CFLAGS: https://github.com/flintlib/flint/blob/main/config/config.guess#L797