BUG: Fix old-style install to prefix top

The old install_files, install_programs, and install_targets commands
used to permit installation to the top of the prefix by specifying
destination '/'.  This was broken in 2.6.0 to 2.6.2 by changes to
enforce valid destinations that did not account for this case.  This
change fixes the case by converting the install destination to '.' which
is the new-style way to specify the top of the installation prefix.

Source/cmInstallFilesCommand.cxx

@@ -125,6 +125,10 @@ void cmInstallFilesCommand::CreateInstallGenerator() const
   // the prefix.  We skip the leading slash given by the user.
   std::string destination = this->Destination.substr(1);
   cmSystemTools::ConvertToUnixSlashes(destination);
+  if(destination.empty())
+    {
+    destination = ".";
+    }
 
   // Use a file install generator.
   const char* no_permissions = "";

Source/cmInstallProgramsCommand.cxx

@@ -86,6 +86,10 @@ void cmInstallProgramsCommand::FinalPass()
   // the prefix.  We skip the leading slash given by the user.
   std::string destination = this->Destination.substr(1);
   cmSystemTools::ConvertToUnixSlashes(destination);
+  if(destination.empty())
+    {
+    destination = ".";
+    }
 
   // Use a file install generator.
   const char* no_permissions = "";

Source/cmLocalGenerator.cxx

@@ -2377,6 +2377,10 @@ cmLocalGenerator
       // We also skip over the leading slash given by the user.
       std::string destination = l->second.GetInstallPath().substr(1);
       cmSystemTools::ConvertToUnixSlashes(destination);
+      if(destination.empty())
+        {
+        destination = ".";
+        }
 
       // Generate the proper install generator for this target type.
       switch(l->second.GetType())