install: Support generator expressions in FILES and PROGRAMS mode

Teach the install(FILES) and install(PROGRAMS) commands to evaluate
generator expressions in the list of files.

Extend the ExportImport test to cover installation cases involving
generator expressions.

Source/cmInstallFilesGenerator.h

@@ -34,6 +34,11 @@ public:
 
 protected:
   virtual void GenerateScriptActions(std::ostream& os, Indent const& indent);
+  virtual void GenerateScriptForConfig(std::ostream& os,
+                                       const char* config,
+                                       Indent const& indent);
+  void AddFilesInstallRule(std::ostream& os, Indent const& indent,
+                           std::vector<std::string> const& files);
 
   cmMakefile* Makefile;
   std::vector<std::string> Files;