cmMakefile: Store container of cmExportBuildFileGenerators.

Set a cmLocalGenerator on each instance at compute time.  That will
soon be needed to access cmGeneratorTarget instances.

If a cmExportBuildFileGenerator is processed early during configure time as a
result of CMP0024 it must be removed from the list to process later at generate
time.

Source/cmExportBuildFileGenerator.h

@@ -43,10 +43,7 @@ public:
   /** Set whether to append generated code to the output file.  */
   void SetAppendMode(bool append) { this->AppendMode = append; }
 
-  void SetMakefile(cmMakefile *mf) {
-    this->Makefile = mf;
-    this->Backtrace = this->Makefile->GetBacktrace();
-  }
+  void Compute(cmLocalGenerator* lg);
 
 protected:
   // Implement virtual methods from the superclass.
@@ -80,8 +77,7 @@ protected:
   std::vector<std::string> Targets;
   cmExportSet *ExportSet;
   std::vector<cmGeneratorTarget*> Exports;
-  cmMakefile* Makefile;
-  cmListFileBacktrace Backtrace;
+  cmLocalGenerator* LG;
 };
 
 #endif