cmake: add pre-fill for Unix, enable in GHA/macos, verify pre-fills

[vszakats]

TL;DR: Save 10 minutes of CI time for GHA/macos jobs using pre-fills and
add pre-fill verification for Apple and Windows. Also restores Xcode job
and saves 1.5-10 minutes configuring iOS jobs.

Pre-filling feature detection results can bring down the CMake configure
step to ~5 seconds on most GHA runners, ~10 seconds in slow envs like
Cygwin/MSYS2.

The potential savings per job are:

• 5-40 (average 19) seconds on GHA/macos (33 jobs)
• ~10 seconds on GHA for iOS GNU Makefile (1 job)
• 1.5-10 minutes on GHA for iOS Xcode generator (1 job)
• 10 seconds on GHA/linux with native Ubuntu (12 jobs)
• 40 seconds for Cygwin/MSYS2 (2 jobs)
• 5-10 seconds for virtualized BSDs, native CPU (3 jobs)
• ~60 seconds for virtualized BSDs, emulated CPU (1 job)

On native Windows pre-filling has been in place for a long time and
saving 8 minutes (VS2019-VS2015) to 1.5-2 minutes (VS2022), 3 minutes
(VS2022 UWP), and 30-60 seconds (MinGW), per CI job.

The downside is that detection results need to be manually collected and
filtered to those that universally apply to all platforms that they are
enabled on. Another downside is that by using a cache, we're not running
the actual detections, and thus won't catch regressions in them. It
means we must make sure that the cache is solid and matches with actual
detections results. An upside is that it gives a rough overview of which
features are available on which platforms. Another upside is pre-filled
values do work for feature detections skipped for cross-builds, e.g.
HAVE_WRITABLE_ARGV.

This PR adds a pre-fill cache that supports all Unixes (except OmniOS)
used in CI, and makes it usable with an internal option. It also enables
it for GHA/macos CI jobs, where the maximum savings are. And also for
the two iOS [1] and two Cygwin/MSYS2 jobs. The latters don't have
pre-fill checks and we can drop them if they turn into a hassle.

Saving:

• 10 minutes of CI time per GHA/macos workflow run. [2]
• ~80 seconds per GHA/windows workflow run with Cygwin/MSYS2.
  (offsetting the cost of pre-fill verifications)
• 1.5-10 minutes per GHA/non-native runs with iOS jobs. [3]

You can enable pre-fill locally with -D_CURL_PREFILL=ON. It's
experimental, and if you experience a problem, file a PR or an Issue.

This PR also adds a pre-fill checker for macOS and MinGW/MSVC Windows
GHA jobs to catch if the cache diverges from real detections. It also
adds this logic to AppVeyor, but doesn't enable it due to the perf
penalty of 2 minutes mininum.

The pre-fill checker works by configuring out-of-tree with and without
pre-fill, then diffing their lib/curl_config.h outputs.

Exceptions are 3 detection results exposed indirectly [4], and missing
to expose 2, of which one is the C89 header stddef.h. While we assume
the C99 stdint.h available outside iOS. We can expose them in the
future, if necessary.

The pre-fill checks cost in total:

• ~20 seconds for macOS
• ~40 seconds for MinGW on GHA
• ~80 seconds for MSVC on GHA (UWP would be 2x this)

An extra time saving potential is caching type sizes. They are
well-known, and seldom change, esp. in CI. GHA/Windows jobs spend 8-17
seconds per job on these ~12 feature checks. ~5s on Cygwin/MSYS2. Couple
of seconds on other platforms. (This PR doesn't make this optimization.)

Another opportunity is doing the same for autotools, which typically
spends more time in the configuration step than cmake.

[1] Xcode job restored as a
follow-up to be5f202 #16302

[2] GHA/macos cmake configure times in seconds:

Job	Bef.	After	Gain
CM clang GnuTLS !ldap krb5	21.2	4.5	16.7
CM clang LibreSSL !ldap heimdal c-ares +examples	13.3	3.9	9.4
CM clang OpenSSL +static libssh +examples	20.0	4.6	15.4
CM clang OpenSSL IDN clang-tidy~ (w/chkprefill)	15.7	18.6	-2.9
CM clang OpenSSL gsasl rtmp AppleIDN	25.0	4.7	20.3
CM clang OpenSSL torture !FTP	15.3	4.5	10.8
CM clang OpenSSL torture FTP	25.0	5.9	19.1
CM clang SecureTransport debug	18.0	3.8	14.2
CM clang macos-13 SecureTransport	45.8	12.4	33.4
CM clang macos-14 SecureTransport	15.8	4.6	11.2
CM clang macos-15 SecureTransport	26.8	6.1	20.7
CM clang mbedTLS openldap brotli zstd	15.1	6.5	8.6
CM clang wolfSSL !ldap brotli zstd	27.0	4.4	22.6
CM gcc-12 GnuTLS !ldap krb5	39.1	8.7	30.4
CM gcc-12 LibreSSL !ldap heimdal c-ares +examples	23.8	7.2	16.6
CM gcc-12 OpenSSL +static libssh +examples	20.7	8.5	12.2
CM gcc-12 OpenSSL gsasl rtmp AppleIDN	23.1	10.1	13.0
CM gcc-12 SecureTransport debug	21.1	4.8	16.3
CM gcc-12 mbedTLS openldap brotli zstd	21.4	5.8	15.6
CM gcc-12 wolfSSL !ldap brotli zstd	21.1	6.9	14.2
CM gcc-14 macos-13 SecureTransport	61.9	18.7	43.2
CM gcc-14 macos-14 SecureTransport	30.5	6.4	24.1
CM gcc-14 macos-15 SecureTransport	32.7	8.4	24.3
CM llvm@15 GnuTLS !ldap krb5	21.1	7.5	13.6
CM llvm@15 LibreSSL !ldap heimdal c-ares +exampl~	24.6	6.8	17.8
CM llvm@15 OpenSSL +static libssh +examples	19.0	6.4	12.6
CM llvm@15 OpenSSL gsasl rtmp AppleIDN	19.0	8.2	10.8
CM llvm@15 SecureTransport debug	18.0	5.4	12.6
CM llvm@15 macos-13 SecureTransport	66.2	25.7	40.5
CM llvm@15 macos-14 SecureTransport	31.9	6.1	25.8
CM llvm@15 mbedTLS openldap brotli zstd	19.5	8.9	10.6
CM llvm@15 wolfSSL !ldap brotli zstd	24.3	5.9	18.4
CM llvm@18 macos-15 SecureTransport	33.8	6.4	27.4
Total	856.8	257.3	599.5
Before: https://github.com/curl/curl/actions/runs/13311042735/job/37173478424
After: https://github.com/curl/curl/actions/runs/13313927119/job/37183206426?pr=15841

[3] iOS:
Before: https://github.com/curl/curl/actions/runs/13326401704?pr=15841
After: https://github.com/curl/curl/actions/runs/13332177764?pr=15841

[4] detection results exposed indirectly in curl_config.h:

• HAVE_FILE_OFFSET_BITS via _FILE_OFFSET_BITS
• HAVE_GETHOSTBYNAME_R_*_REENTRANT via NEED_REENTRANT
• HAVE_SOCKADDR_IN6_SIN6_ADDR via USE_IPV6

---

w/o ws: https://github.com/curl/curl/pull/15841/files?w=1

• add a CI check comparing the macOS cached values with actual detection results (possibly to configure-vs-cmake).
• leave a real macOS job as a canary that doesn't use caching. [SKIP, comparison is enough]
• omit caching if any of the CMake sources changes. [SKIP]
• keep caching only on macOS, where the vast majority of time saving is. leave the rest disable by default in CI. (but keep the logic, because it can still be beneficial locally)
• consider uploading caches for MS-DOS/AmigaOS. [FUTURE PR maybe, useful for local development, but it's probably not of wide interest.]
• try detecting "old" Linux, via glibc version, automatically? [FUTURE PR if this looks interesting, or perhaps affects more than a single detection result]
• add script to generate a cache from CI log output (or CMakeCache.txt). [SKIP, the pre-fill checker output should be enough.]
• enable pre-fill checker in MinGW and MSVC GHA/windows jobs.