Genex: Add more extensive support for an unbounded number of parameters

Previously genex support for unbounded number of parameters required
at least 1 parameter. We now support zero or more parameters, and two
or more parameters.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -166,9 +166,13 @@ std::string GeneratorExpressionContent::EvaluateParameters(
     reportError(context, this->GetOriginalExpression(),
                 "$<" + identifier +
                   "> expression requires at least one parameter.");
-  }
-  if (numExpected == cmGeneratorExpressionNode::OneOrZeroParameters &&
-      parameters.size() > 1) {
+  } else if (numExpected == cmGeneratorExpressionNode::TwoOrMoreParameters &&
+             parameters.size() < 2) {
+    reportError(context, this->GetOriginalExpression(),
+                "$<" + identifier +
+                  "> expression requires at least two parameters.");
+  } else if (numExpected == cmGeneratorExpressionNode::OneOrZeroParameters &&
+             parameters.size() > 1) {
     reportError(context, this->GetOriginalExpression(),
                 "$<" + identifier +
                   "> expression requires one or zero parameters.");

Source/cmGeneratorExpressionNode.h

@@ -20,7 +20,9 @@ struct cmGeneratorExpressionNode
   {
     DynamicParameters = 0,
     OneOrMoreParameters = -1,
-    OneOrZeroParameters = -2
+    TwoOrMoreParameters = -2,
+    ZeroOrMoreParameters = -3,
+    OneOrZeroParameters = -4
   };
   virtual ~cmGeneratorExpressionNode() = default;