Handle colons as a special case in the generator expression parser.

Like the special case for commas, this ensures that the colon only has
special meaning as the delimiter between the identifier and the
parameters of a particular expression, but constructs such as

 INCLUDE_DIRECTORIES "$<1:C:\foo>"

are legal.

Source/cmGeneratorExpressionParser.cxx

@@ -127,6 +127,11 @@ void cmGeneratorExpressionParser::ParseGeneratorExpression(
         parameters.resize(parameters.size() + 1);
         ++this->it;
         }
+      if (this->it->TokenType == cmGeneratorExpressionToken::ColonSeparator)
+        {
+        extendText(*(parameters.end() - 1), this->it);
+        ++this->it;
+        }
       if (this->it == this->Tokens.end())
         {
         break;