preparser: never call the external preparser on unsupported platforms

And log an error in an external processing is requested on an unsupported platform.

We fallback to internal processing. Otherwise the preparsing will always fail.
And this may block (lib)vlc.

src/preparser/preparser.c

@@ -40,7 +40,17 @@ vlc_preparser_t *vlc_preparser_New(vlc_object_t *obj,
     if (preparser == NULL) {
         return NULL;
     }
+#if defined(HAVE_VLC_PROCESS_SPAWN)
+    const bool external_process = cfg->external_process;
+#else
     if (cfg->external_process) {
+        assert(!cfg->external_process);
+        msg_Err(obj, "external preparser requested on unsupported platform");
+        return NULL;
+    }
+    const bool external_process = false;
+#endif
+    if (external_process) {
         preparser->sys = vlc_preparser_external_New(preparser, obj, cfg);
     } else {
         preparser->sys = vlc_preparser_internal_New(preparser, obj, cfg);