cmake: support building some complicated examples, build them in CI

Build these examples when the necessary dependencies are present:
- cacertinmem, usercertinmem (OpenSSL/fork)
- multi-uv (libuv)
- multithread, threaded-ssl (pthread)
- sessioninfo (GnuTLS)

Indicate the necessary dependency via a `Required:` comment placed in
the source file. A single dependency per source is supported as of now.
The name of the dependency should match the variable used within
the cmake scripts, which in turn matches the macro used in the config
header. E.g. for GnuTLS it's `USE_GNUTLS`.

Also:
- GHA/macos: build examples in two job to test GnuTLS and pthread ones.
- GHA/linux: enable libuv to test it with examples.

Follow-up to 6bb7714032 #18914
Closes #18909

.github/workflows/macos.yml

@@ -276,7 +276,7 @@ jobs:
             configure: --enable-debug --with-openssl=/opt/homebrew/opt/openssl
             tflags: --test-event
           # cmake
-          - name: 'OpenSSL gsasl rtmp AppleIDN SecTrust'
+          - name: 'OpenSSL gsasl rtmp AppleIDN SecTrust +examples'
             install: libnghttp3 libngtcp2 gsasl rtmpdump
             generate: -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DCURL_USE_GSASL=ON -DUSE_LIBRTMP=ON -DUSE_APPLE_IDN=ON -DUSE_NGTCP2=ON -DCURL_DISABLE_VERBOSE_STRINGS=ON -DUSE_APPLE_SECTRUST=ON
           - name: 'MultiSSL AppleIDN clang-tidy +examples'
@@ -314,7 +314,7 @@ jobs:
             install: brotli mbedtls zstd
             install_steps: codeset-test
             generate: -DCURL_USE_MBEDTLS=ON -DCURL_DISABLE_LDAP=ON -DCURL_DEFAULT_SSL_BACKEND=mbedtls -DCURL_USE_OPENSSL=ON -DUSE_APPLE_IDN=ON
-          - name: 'GnuTLS !ldap krb5'
+          - name: 'GnuTLS !ldap krb5 +examples'
             install: gnutls nettle krb5
             generate: -DENABLE_DEBUG=ON -DCURL_USE_GNUTLS=ON -DCURL_USE_OPENSSL=OFF -DCURL_USE_GSSAPI=ON -DGSS_ROOT_DIR=/opt/homebrew/opt/krb5 -DCURL_DISABLE_LDAP=ON -DUSE_SSLS_EXPORT=ON
           - name: 'aws-lc'