add_custom_{command,target}: Add genex support to OUTPUT and BYPRODUCTS

Move rejection of `#`, `<`, and `>` characters in outputs and byproducts
to a generate-time check.  This removes the front-end check that
disallowed generator expressions.  The generators have already been
updated to handle them.

Fixes: #12877

Source/cmAddCustomCommandCommand.cxx

@@ -5,7 +5,6 @@
 #include <sstream>
 #include <unordered_set>
 
-#include "cmCheckCustomOutputs.h"
 #include "cmCustomCommand.h"
 #include "cmCustomCommandLines.h"
 #include "cmCustomCommandTypes.h"
@@ -298,13 +297,6 @@ bool cmAddCustomCommandCommand(std::vector<std::string> const& args,
     return false;
   }
 
-  // Make sure the output names and locations are safe.
-  if (!cmCheckCustomOutputs(output, "OUTPUT", status) ||
-      !cmCheckCustomOutputs(outputs, "OUTPUTS", status) ||
-      !cmCheckCustomOutputs(byproducts, "BYPRODUCTS", status)) {
-    return false;
-  }
-
   // Check for an append request.
   if (append) {
     mf.AppendCustomCommandToOutput(output[0], depends, implicit_depends,