build: fix compiler warnings with auths disabled

```
./curl/lib/http.c:737:12: warning: unused variable 'result' [-Wunused-variable]
  CURLcode result = CURLE_OK;
           ^
./curl/lib/http.c:1143:12: warning: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Wextra-semi-stmt]
           ;
           ^
./curl/lib/http.c:995:18: warning: variable 'availp' set but not used [-Wunused-but-set-variable]
  unsigned long *availp;
                 ^
./curl/lib/http.c:996:16: warning: variable 'authp' set but not used [-Wunused-but-set-variable]
  struct auth *authp;
               ^
./curl/lib/http.c:979:12: warning: unused function 'is_valid_auth_separator' [-Wunused-function]
static int is_valid_auth_separator(char ch)
           ^
5 warnings generated.
```

Follow-up to 0d3956b curl#11895
Follow-up to e92edfb curl#11490

Closes #xxxxx

lib/http.c

@@ -845,7 +845,7 @@ output_auth_headers(struct Curl_easy *data,
   else
     authstatus->multipass = FALSE;
 
-  return CURLE_OK;
+  return result;
 }
 
 /**
@@ -970,17 +970,21 @@ Curl_http_output_auth(struct Curl_easy *data,
 }
 #endif
 
-/*
- * Curl_http_input_auth() deals with Proxy-Authenticate: and WWW-Authenticate:
- * headers. They are dealt with both in the transfer.c main loop and in the
- * proxy CONNECT loop.
- */
-
+#if defined(USE_SPNEGO) || defined(USE_NTLM) || \
+  !defined(CURL_DISABLE_DIGEST_AUTH) || \
+  !defined(CURL_DISABLE_BASIC_AUTH) || \
+  !defined(CURL_DISABLE_BEARER_AUTH)
 static int is_valid_auth_separator(char ch)
 {
   return ch == '\0' || ch == ',' || ISSPACE(ch);
 }
+#endif
 
+/*
+ * Curl_http_input_auth() deals with Proxy-Authenticate: and WWW-Authenticate:
+ * headers. They are dealt with both in the transfer.c main loop and in the
+ * proxy CONNECT loop.
+ */
 CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
                               const char *auth) /* the first non-space */
 {
@@ -995,7 +999,10 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
   unsigned long *availp;
   struct auth *authp;
 
-  (void) conn; /* In case conditionals make it unused. */
+  /* In case conditionals make them unused. */
+  (void) conn;
+  (void) availp;
+  (void) authp;
 
   if(proxy) {
     availp = &data->info.proxyauthavail;
@@ -1046,9 +1053,9 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
         }
       }
     }
-    else
 #endif
 #ifdef USE_NTLM
+    else
       /* NTLM support requires the SSL crypto libs */
       if(checkprefix("NTLM", auth) && is_valid_auth_separator(auth[4])) {
         if((authp->avail & CURLAUTH_NTLM) ||
@@ -1085,9 +1092,9 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
           }
         }
       }
-      else
 #endif
 #ifndef CURL_DISABLE_DIGEST_AUTH
+      else
         if(checkprefix("Digest", auth) && is_valid_auth_separator(auth[6])) {
           if((authp->avail & CURLAUTH_DIGEST) != 0)
             infof(data, "Ignoring duplicate digest auth header.");
@@ -1108,9 +1115,9 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
             }
           }
         }
-        else
 #endif
 #ifndef CURL_DISABLE_BASIC_AUTH
+        else
           if(checkprefix("Basic", auth) &&
              is_valid_auth_separator(auth[5])) {
             *availp |= CURLAUTH_BASIC;
@@ -1124,9 +1131,9 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
               data->state.authproblem = TRUE;
             }
           }
-          else
 #endif
 #ifndef CURL_DISABLE_BEARER_AUTH
+          else
             if(checkprefix("Bearer", auth) &&
                is_valid_auth_separator(auth[6])) {
               *availp |= CURLAUTH_BEARER;
@@ -1139,8 +1146,6 @@ CURLcode Curl_http_input_auth(struct Curl_easy *data, bool proxy,
                 data->state.authproblem = TRUE;
               }
             }
-#else
-           ;
 #endif
 
     /* there may be multiple methods on one line, so keep reading */