check if subsystem was specified

src/linker/lnk.c

@@ -3383,6 +3383,11 @@ lnk_run(int argc, char **argv)
           }
         }
 
+        // do we have a subsystem?
+        if (config->subsystem == PE_WindowsSubsystem_UNKNOWN) {
+          lnk_error(LNK_Error_NoSubsystem, "unknown subsystem, please use /SUBSYSTEM to set subsytem type you need");
+        }
+      
         if (config->subsystem_ver.major == 0 && config->subsystem_ver.minor == 0) {
           // subsystem version not specified, set default values
           config->subsystem_ver = lnk_get_default_subsystem_version(config->subsystem, config->machine);