Disabled open_file_cache on platforms without pread().

Current open file cache code cannot properly work on platforms without
pread(), since file->sys_offset is not shared across files.  Further, it
is not set on file initialization after ngx_open_cached_file(), leading
to incorrect value 0 instead of non-zero current offset for cached file
descriptors.

Since platforms without pread() are rather exotic nowadays, fix is to
disable open_file_cache for them.

Author: mdounin

src/http/ngx_http_core_module.c

@@ -4981,6 +4981,8 @@ ngx_http_core_error_page(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
 static char *
 ngx_http_core_open_file_cache(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
 {
+#if (NGX_HAVE_PREAD || NGX_WIN32)
+
     ngx_http_core_loc_conf_t *clcf = conf;
 
     time_t       inactive;
@@ -5048,11 +5050,17 @@ ngx_http_core_open_file_cache(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
     }
 
     clcf->open_file_cache = ngx_open_file_cache_init(cf->pool, max, inactive);
-    if (clcf->open_file_cache) {
-        return NGX_CONF_OK;
+    if (clcf->open_file_cache == NULL) {
+        return NGX_CONF_ERROR;
     }
 
-    return NGX_CONF_ERROR;
+#else
+    ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                       "\"open_file_cache\" is not supported "
+                       "on this platform, ignored");
+#endif
+
+    return NGX_CONF_OK;
 }