cmPlaceHolderExpander: Add recognition of genex

Add a mode to ensure genex at kept as is.

Source/cmPlaceholderExpander.cxx

@@ -4,7 +4,8 @@
 
 #include "cmsys/String.h"
 
-std::string& cmPlaceholderExpander::ExpandVariables(std::string& s)
+std::string& cmPlaceholderExpander::ExpandVariables(std::string& s,
+                                                    HandleGenex handleGenex)
 {
   std::string::size_type start = s.find('<');
   // no variables to expand
@@ -14,6 +15,13 @@ std::string& cmPlaceholderExpander::ExpandVariables(std::string& s)
   std::string::size_type pos = 0;
   std::string expandedInput;
   while (start != std::string::npos && start < s.size() - 2) {
+    if (handleGenex == HandleGenex::Yes && start != 0 && s[start - 1] == '$') {
+      // this is a generator expression
+      // skip it and try to find the next < in the string
+      start = s.find('<', start + 1);
+      continue;
+    }
+
     std::string::size_type end = s.find('>', start);
     // if we find a < with no > we are done
     if (end == std::string::npos) {

Source/cmPlaceholderExpander.h

@@ -10,9 +10,16 @@
 class cmPlaceholderExpander
 {
 public:
+  enum class HandleGenex
+  {
+    No,
+    Yes
+  };
+
   virtual ~cmPlaceholderExpander() = default;
 
-  std::string& ExpandVariables(std::string& string);
+  std::string& ExpandVariables(std::string& string,
+                               HandleGenex handleGenex = HandleGenex::No);
 
 protected:
   virtual std::string ExpandVariable(std::string const& variable) = 0;