Re: Broken build on macOS (Universal / Intel): cpuid instruction not available

I wrote:
> Also, the reason we manage to set USE_AVX2_WITH_RUNTIME_CHECK
> in a multi-arch build is that what configure actually tests
> is (a) is $host_cpu x86_64 and (b) does the compiler accept
> __attribute__((target("avx2"))). On a multi-arch build,
> the ARM side spits out a warning
> '+avx2' is not a recognized feature for this target (ignoring feature)
> but it doesn't actually fail, so configure thinks that's a success.

After further contemplation, I think the best fix for this problem
is to undo configure's mistake globally, not just in checksum.c.
checksum.c is the only file using USE_AVX2_WITH_RUNTIME_CHECK today,
but that might not be true forever. Also, I think it's pure chance
that the test for that symbol succeeds in a universal build while
other similar tests don't. So what the attached v2 does is to
#undef USE_AVX2_WITH_RUNTIME_CHECK as well as the USE_AVX512
symbols in c.h, if we're not in fact building for x86_64 hardware.
We could extend the list of things to #undef there later, if it
proves necessary.

The shape of this patch is partially dictated by not wanting to
create problems for Munro's pending patch [1], but I think it's
a reasonable solution even without that consideration.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGL8Hs-phHPugrWM%3D5dAkcT897rXyazYzLw-Szxnzgx-rA%40mail.gmail.com