gssapi: add support for Apple GSS Framework

Add support for the native GSS Framework on Apple targets, which is
Apple's fork of Heimdal. This option allows to drop Apple's deprecated
MIT Kerberos compatibility shim "mit-krb5/1.7-prerelease".

Source code uses different headers, other than that no source changes
are necessary.

You can enable by configuring with:
autotools: `--enable-gssapi-apple`
CMake: `-DCURL_USE_GSSAPI=ON -DCURL_GSS_FLAVOR=Apple`

These options are experimental, and may receive breaking updates till
the GSS backend selection logic settles, for Apple and also for the GNU
and MIT implementation.

Version string:
```
curl 8.21.0-DEV (aarch64-apple-darwin25.4.0) libcurl/8.21.0-DEV
  OpenSSL/3.6.2 zlib/1.2.12 AppleIDN AppleGSS OpenLDAP/2.4.28/Apple
```

Also:
- drop in-source deprecation warning suppressions when using AppleGSS.
- GHA/macos: enable Apple GSS in CI jobs.

Supported by:
iOS 5.0+, iPadOS 5.0+, Mac Catalyst 13.0+, macOS 10.14+, visionOS 1.0+

Ref: https://developer.apple.com/documentation/gss
Ref: #19109

Closes #22052

.github/workflows/macos.yml

@@ -39,7 +39,7 @@ permissions: {}
 # - 10.9   Mavericks (2013)     - LDAP (build-time, deprecated), memset_s(), OCSP (runtime)
 # - 10.11  El Capitan (2015)    - connectx() (runtime)
 # - 10.12  Sierra (2016)        - clock_gettime() (build-time, runtime)
-# - 10.14  Mojave (2018)        - SecTrustEvaluateWithError() (runtime)
+# - 10.14  Mojave (2018)        - SecTrustEvaluateWithError() (runtime), GSS Framework
 
 env:
   CURL_CI: github
@@ -328,12 +328,12 @@ jobs:
               -DCURL_CLANG_TIDY=ON -DCLANG_TIDY=/opt/homebrew/opt/llvm/bin/clang-tidy
               -DCURL_ENABLE_NTLM=ON -DUSE_PROXY_HTTP3=ON
 
-          - name: 'LibreSSL openldap krb5 c-ares +examples'
+          - name: 'LibreSSL openldap AppleGSS c-ares +examples'
             compiler: clang
             install: libressl krb5 openldap
             generate: >-
-              -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/libressl -DENABLE_ARES=ON -DCURL_USE_GSSAPI=ON
-              -DGSS_ROOT_DIR=/opt/homebrew/opt/krb5
+              -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/libressl -DENABLE_ARES=ON
+              -DCURL_USE_GSSAPI=ON -DCURL_GSS_FLAVOR=Apple
               -DLDAP_INCLUDE_DIR=/opt/homebrew/opt/openldap/include
               -DLDAP_LIBRARY=/opt/homebrew/opt/openldap/lib/libldap.dylib
               -DLDAP_LBER_LIBRARY=/opt/homebrew/opt/openldap/lib/liblber.dylib
@@ -737,6 +737,7 @@ jobs:
               -DUSE_NGHTTP2=OFF -DUSE_LIBIDN2=OFF \
               -DCURL_USE_LIBPSL=OFF -DCURL_USE_LIBSSH2=OFF \
               -DUSE_APPLE_IDN=ON -DUSE_APPLE_SECTRUST=ON \
+              -DCURL_USE_GSSAPI=ON -DCURL_GSS_FLAVOR=Apple \
               ${options}
           else
             export CFLAGS
@@ -756,6 +757,7 @@ jobs:
               --without-nghttp2 --without-libidn2 \
               --without-libpsl \
               --with-apple-idn --with-apple-sectrust \
+              --enable-gssapi-apple \
               ${options}
           fi