ENH: Make dir content cache work during configure

Previously the cmGlobalGenerator::GetDirectoryContent method would work
safely only during build system generation.  These changes make it safe
to use during each configure step by flushing it at the beginning.

Source/cmGlobalGenerator.cxx

@@ -731,6 +731,7 @@ void cmGlobalGenerator::Configure()
   this->LocalGeneratorToTargetMap.clear();
   this->ProjectMap.clear();
   this->RuleHashes.clear();
+  this->DirectoryContentMap.clear();
 
   // start with this directory
   cmLocalGenerator *lg = this->CreateLocalGenerator();

Source/cmGlobalGenerator.h

@@ -213,12 +213,12 @@ public:
       configuration.  This is valid during generation only.  */
   cmTargetManifest const& GetTargetManifest() { return this->TargetManifest; }
 
-  /** Get the content of a directory on disk including the target
-      files to be generated.  This may be called only during the
-      generation step.  It is intended for use only by
-      cmComputeLinkInformation.  */
+  /** Get the content of a directory.  Directory listings are loaded
+      from disk at most once and cached.  During the generation step
+      the content will include the target files to be built even if
+      they do not yet exist.  */
   std::set<cmStdString> const& GetDirectoryContent(std::string const& dir,
-                                                   bool needDisk);
+                                                   bool needDisk = true);
 
   void AddTarget(cmTargets::value_type &v);