windows: add build option to use the native CA store

With the same semantics as Apple SecTrust, in both libcurl and the curl
tool, when using non-Schannel TLS backends. In practice it means that
it makes TLS work without manually or implicitly configuring a CA bundle
`.crt` file, such as `curl-ca-bundle.crt`.

To enable:
- autotools: `--enable-ca-native`
- cmake: `-DCURL_CA_NATIVE=ON`
- CPPFLAGS: `-DCURL_CA_NATIVE`

When enabled:
- enables `CURLSSLOPT_NATIVE_CA` (libcurl) / `--ca-native`
  and `--proxy-ca-native` (curl tool) options by default.
- unsafe search for an on-disk CA bundle gets disabled by default.
  Equivalent to `--disable-ca-search` with autotools,
  `-DCURL_DISABLE_CA_SEARCH=ON` with CMake.
- build-time detection of CA bundle and CA path gets disabled. As with
  Apple SecTrust. This was already the default for Windows.
- native CA can be disabled at run-time with the `--no-ca-native`
  and/or `--no-proxy-ca-native` command-line options.

Rationale: This build option:
- has a repeat and active interest from packagers and users.
- helps integrating curl with Windows for those who need this.
- it also applies to macOS: #17525
  Shipped in curl 8.17.0.
- makes it trivial to use custom certs configured on the OS.
- frees applications/packagers/users from the task of securely
  distributing, and keeping up-to-date, a CA bundle.
- frees potentially many curl tool from configuring a CA bundle manually
  to access HTTPS (and other TLS) URLs. This is traditionally difficult
  on Windows because there is no concept of a universal, protected,
  non-world-writable, location on the file system to securely store
  a CA bundle.
- allows using modern features regardless of Windows version. Some of
  these features are not supported with Schannel (e.g. HTTP/3, ECH) on
  any Windows version.
- is necessary for HTTP/3 builds, where bootstrapping a CA bundle is not
  possible with Schannel, because MultiSSL is not an option, and HTTP/3
  is not supported with Schannel.

Ref: #16181 (previous attempt)
Ref: https://github.com/curl/curl/discussions/9348
Ref: https://github.com/curl/curl/issues/9350
Ref: https://github.com/curl/curl/pull/13111
Ref: https://github.com/microsoft/vcpkg/pull/46459#issuecomment-3162068701
Ref: 22652a5a4c #14582
Ref: eefd03c572 #18703

Closes #18279

lib/curl_config-cmake.h.in

@@ -34,6 +34,9 @@
 /* Default SSL backend */
 #cmakedefine CURL_DEFAULT_SSL_BACKEND "${CURL_DEFAULT_SSL_BACKEND}"
 
+/* Use native CA store */
+#cmakedefine CURL_CA_NATIVE 1
+
 /* disables alt-svc */
 #cmakedefine CURL_DISABLE_ALTSVC 1
 

lib/version.c

@@ -513,9 +513,13 @@ static const struct feat features_table[] = {
 #ifdef USE_LIBPSL
   FEATURE("PSL",         NULL,                CURL_VERSION_PSL),
 #endif
+#ifdef USE_SSL
 #ifdef USE_APPLE_SECTRUST
   FEATURE("AppleSecTrust", NULL,              0),
+#elif defined(CURL_CA_NATIVE)
+  FEATURE("NativeCA",    NULL,              0),
 #endif
+#endif /* USE_SSL */
 #ifdef USE_SPNEGO
   FEATURE("SPNEGO",      NULL,                CURL_VERSION_SPNEGO),
 #endif

lib/vtls/vtls.c

@@ -295,7 +295,7 @@ CURLcode Curl_ssl_easy_config_complete(struct Curl_easy *data)
 #endif
 
   if(Curl_ssl_backend() != CURLSSLBACKEND_SCHANNEL) {
-#ifdef USE_APPLE_SECTRUST
+#if defined(USE_APPLE_SECTRUST) || defined(CURL_CA_NATIVE)
     if(!sslc->custom_capath && !sslc->custom_cafile && !sslc->custom_cablob)
       sslc->native_ca_store = TRUE;
 #endif
@@ -341,7 +341,7 @@ CURLcode Curl_ssl_easy_config_complete(struct Curl_easy *data)
 #ifndef CURL_DISABLE_PROXY
   sslc = &data->set.proxy_ssl;
   if(Curl_ssl_backend() != CURLSSLBACKEND_SCHANNEL) {
-#ifdef USE_APPLE_SECTRUST
+#if defined(USE_APPLE_SECTRUST) || defined(CURL_CA_NATIVE)
     if(!sslc->custom_capath && !sslc->custom_cafile && !sslc->custom_cablob)
       sslc->native_ca_store = TRUE;
 #endif