schannel: disable client cert option if APIs not available

Original MinGW targets Windows 2000 by default, which lacks some APIs and
definitions for this feature. Disable it if these APIs are not available.

Closes https://github.com/curl/curl/pull/2522

lib/vtls/schannel.c

@@ -90,11 +90,17 @@
 #endif
 #endif
 
+#if defined(CryptStringToBinary) && defined(CRYPT_STRING_HEX)
+#define HAS_CLIENT_CERT_PATH
+#endif
+
+#ifdef HAS_CLIENT_CERT_PATH
 #ifdef UNICODE
 #define CURL_CERT_STORE_PROV_SYSTEM CERT_STORE_PROV_SYSTEM_W
 #else
 #define CURL_CERT_STORE_PROV_SYSTEM CERT_STORE_PROV_SYSTEM_A
 #endif
+#endif
 
 #ifndef SP_PROT_SSL2_CLIENT
 #define SP_PROT_SSL2_CLIENT             0x00000008
@@ -199,6 +205,7 @@ set_ssl_version_min_max(SCHANNEL_CRED *schannel_cred, struct connectdata *conn)
   return CURLE_OK;
 }
 
+#ifdef HAS_CLIENT_CERT_PATH
 static CURLcode
 get_cert_location(TCHAR *path, DWORD *store_name, TCHAR **store_path,
                   TCHAR **thumbprint)
@@ -248,6 +255,7 @@ get_cert_location(TCHAR *path, DWORD *store_name, TCHAR **store_path,
 
   return CURLE_OK;
 }
+#endif
 
 static CURLcode
 schannel_connect_step1(struct connectdata *conn, int sockindex)
@@ -401,6 +409,7 @@ schannel_connect_step1(struct connectdata *conn, int sockindex)
       return CURLE_SSL_CONNECT_ERROR;
     }
 
+#ifdef HAS_CLIENT_CERT_PATH
     /* client certificate */
     if(data->set.ssl.cert) {
       DWORD cert_store_name;
@@ -453,6 +462,12 @@ schannel_connect_step1(struct connectdata *conn, int sockindex)
 
       CertCloseStore(cert_store, 0);
     }
+#else
+    if(data->set.ssl.cert) {
+      failf(data, "schannel: client cert support not built in");
+      return CURLE_NOT_BUILT_IN;
+    }
+#endif
 
     /* allocate memory for the re-usable credential handle */
     BACKEND->cred = (struct curl_schannel_cred *)