Breaking change in curl 8.16:
https://github.com/curl/curl/commit/1a12663d06d9c42fd40a8655d5247f7afeeb8e1e

Change curl_proxytype to long.

See also: https://bugs.gentoo.org/962744

--- a/src/VBox/Runtime/generic/http-curl.cpp
+++ b/src/VBox/Runtime/generic/http-curl.cpp
@@ -188,7 +188,7 @@
     /** Proxy port number (UINT32_MAX if not specified). */
     uint32_t            uProxyPort;
     /** The proxy type (CURLPROXY_HTTP, CURLPROXY_SOCKS5, ++). */
-    curl_proxytype      enmProxyType;
+    long      enmProxyType;
     /** Proxy username (RTStrFree). */
     char               *pszProxyUsername;
     /** Proxy password (RTStrFree). */
@@ -591,7 +591,7 @@
  * @param   pszUsername     The proxy username, or NULL if none.
  * @param   pszPassword     The proxy password, or NULL if none.
  */
-static int rtHttpUpdateProxyConfig(PRTHTTPINTERNAL pThis, curl_proxytype enmProxyType, const char *pszHost,
+static int rtHttpUpdateProxyConfig(PRTHTTPINTERNAL pThis, long enmProxyType, const char *pszHost,
                                    uint32_t uPort, const char *pszUsername, const char *pszPassword)
 {
     CURLcode rcCurl;
@@ -610,7 +610,7 @@
     if (   pThis->fReapplyProxyInfo
         || enmProxyType != pThis->enmProxyType)
     {
-        rcCurl = curl_easy_setopt(pThis->pCurl, CURLOPT_PROXYTYPE, (long)enmProxyType);
+        rcCurl = curl_easy_setopt(pThis->pCurl, CURLOPT_PROXYTYPE, enmProxyType);
         AssertMsgReturn(CURL_SUCCESS(rcCurl), ("CURLOPT_PROXYTYPE=%d: %d (%#x)\n", enmProxyType, rcCurl, rcCurl),
                         VERR_HTTP_CURL_PROXY_CONFIG);
         pThis->enmProxyType = enmProxyType;
@@ -871,7 +871,7 @@
             char    *pszPassword = RTUriParsedAuthorityPassword(pszProxyUrl, &Parsed);
             uint32_t uProxyPort  = RTUriParsedAuthorityPort(pszProxyUrl, &Parsed);
             bool     fUnknownProxyType = false;
-            curl_proxytype enmProxyType;
+            long enmProxyType;
             if (RTUriIsSchemeMatch(pszProxyUrl, "http"))
             {
                 enmProxyType  = CURLPROXY_HTTP;
@@ -1352,7 +1352,7 @@
      * Determine the proxy type (not entirely sure about type == proxy type and
      * not scheme/protocol)...
      */
-    curl_proxytype  enmProxyType      = CURLPROXY_HTTP;
+    long  enmProxyType      = CURLPROXY_HTTP;
     uint32_t        uDefaultProxyPort = 8080;
     if (   CFEqual(hStrProxyType, kCFProxyTypeHTTP)
         || CFEqual(hStrProxyType, kCFProxyTypeHTTPS))