Defer check for sources within a target until generation.

The `add_library` and `add_executable` commands can now be called with
no source-files and won't generate a warning or error message, as long
as source-files will be added later via the `target_sources` command.
If during the generation step still no sources are associated with
targets created by such calls a useful error message will be generated
and generation fails.

Targets of type `INTERFACE_LIBRARY`, `UTILITY` or `GLOBAL_TARGET` are
excluded from this check because we do not need sources for these target
types during generation.

Fixes: #16872

Source/cmAddExecutableCommand.cxx

@@ -18,7 +18,7 @@ class cmExecutionStatus;
 bool cmAddExecutableCommand::InitialPass(std::vector<std::string> const& args,
                                          cmExecutionStatus&)
 {
-  if (args.size() < 2) {
+  if (args.empty()) {
     this->SetError("called with incorrect number of arguments");
     return false;
   }
@@ -191,12 +191,6 @@ bool cmAddExecutableCommand::InitialPass(std::vector<std::string> const& args,
     }
   }
 
-  if (s == args.end()) {
-    this->SetError(
-      "called with incorrect number of arguments, no sources provided");
-    return false;
-  }
-
   std::vector<std::string> srclists(s, args.end());
   cmTarget* tgt =
     this->Makefile->AddExecutable(exename.c_str(), srclists, excludeFromAll);