ENH: Teach file(INSTALL) relative paths

This teaches the undocumented file(INSTALL) command to deal with
relative paths.  Relative input file paths are evaluated with respect to
the current source directory.  Relative output file paths are evaluated
with respect to the current binary directory.

While this command is currently used only in cmake_install.cmake scripts
(in -P script mode), this cleans up its interface in preparation for a
documented signature.

Source/cmFileCommand.cxx

@@ -1216,10 +1216,27 @@ bool cmFileCopier::CheckValue(std::string const& arg)
   switch(this->Doing)
     {
     case DoingFiles:
-      this->Files.push_back(arg);
+      if(arg.empty() || cmSystemTools::FileIsFullPath(arg.c_str()))
+        {
+        this->Files.push_back(arg);
+        }
+      else
+        {
+        std::string file = this->Makefile->GetCurrentDirectory();
+        file += "/" + arg;
+        this->Files.push_back(file);
+        }
       break;
     case DoingDestination:
-      this->Destination = arg;
+      if(arg.empty() || cmSystemTools::FileIsFullPath(arg.c_str()))
+        {
+        this->Destination = arg;
+        }
+      else
+        {
+        this->Destination = this->Makefile->GetCurrentOutputDirectory();
+        this->Destination += "/" + arg;
+        }
       this->Doing = DoingNone;
       break;
     case DoingRegex: