cmGeneratorTarget: Implement cmTargetPropertyComputer interface

Populate a local member for the sources property when the instance is
created.  Pass the parameter to avoid the policy check when doing so.
Ordinarily, the GetSources function should not be called unconditionally
(at generate time), but we need to do so here in case the property is
read in a generator expression.  The intent is to be able to implement
cmGeneratorTarget without requiring cmTarget.

Source/cmGeneratorTarget.cxx

@@ -18,6 +18,7 @@
 #include "cmSystemTools.h"
 #include "cmTarget.h"
 #include "cmTargetLinkLibraryType.h"
+#include "cmTargetPropertyComputer.h"
 #include "cm_auto_ptr.hxx"
 #include "cmake.h"
 
@@ -42,6 +43,28 @@
 #define UNORDERED_SET std::set
 #endif
 
+template <>
+const char* cmTargetPropertyComputer::GetSources<cmGeneratorTarget>(
+  cmGeneratorTarget const* tgt, cmMessenger* /* messenger */,
+  cmListFileBacktrace const& /* context */)
+{
+  return tgt->GetSourcesProperty();
+}
+
+template <>
+const char* cmTargetPropertyComputer::ComputeLocationForBuild<
+  cmGeneratorTarget>(cmGeneratorTarget const* tgt)
+{
+  return tgt->GetLocation("");
+}
+
+template <>
+const char* cmTargetPropertyComputer::ComputeLocation<cmGeneratorTarget>(
+  cmGeneratorTarget const* tgt, const std::string& config)
+{
+  return tgt->GetLocation(config);
+}
+
 class cmGeneratorTarget::TargetPropertyEntry
 {
   static cmLinkImplItem NoLinkImplItem;
@@ -320,6 +343,21 @@ cmGeneratorTarget::~cmGeneratorTarget()
   cmDeleteAll(this->LinkInformation);
 }
 
+const char* cmGeneratorTarget::GetSourcesProperty() const
+{
+  std::vector<std::string> values;
+  for (std::vector<cmGeneratorTarget::TargetPropertyEntry *>::const_iterator
+         it = this->SourceEntries.begin(),
+         end = this->SourceEntries.end();
+       it != end; ++it) {
+    values.push_back((*it)->ge->GetInput());
+  }
+  static std::string value;
+  value.clear();
+  value = cmJoin(values, "");
+  return value.c_str();
+}
+
 cmGlobalGenerator* cmGeneratorTarget::GetGlobalGenerator() const
 {
   return this->GetLocalGenerator()->GetGlobalGenerator();

Source/cmGeneratorTarget.h

@@ -532,6 +532,8 @@ public:
 
   std::string GetFortranModuleDirectory(std::string const& working_dir) const;
 
+  const char* GetSourcesProperty() const;
+
 private:
   void AddSourceCommon(const std::string& src);