installer: actually WINDOWSBASISROOTNAME was wrong

What _actually_ determines the prefix of the destination is the HostName
property of the scp2 Directory with Style OFFICEDIRECTORY, so check that.
The two are often the same but not when building a LibreOffice_Dev_SDK.

Change-Id: Ic127447ef40731f0967b89ece631f9c7b076720f

solenv/bin/modules/installer/scriptitems.pm

@@ -1018,7 +1018,8 @@ sub get_Source_Directory_For_Files_From_Includepathlist
 
     installer::logger::include_header_into_logfile("$item:");
 
-    my $extrarootdir = $allvariables->{'WINDOWSBASISROOTNAME'};
+    my ($foundit, $dontcare, $extrarootdir) =
+        get_office_directory_gid_and_hostname($dirsref);
     my $infoline = "";
 
     for ( my $i = 0; $i <= $#{$filesarrayref}; $i++ )
@@ -1045,7 +1046,7 @@ sub get_Source_Directory_For_Files_From_Includepathlist
         my $instdirdestination;
         if ($destination)
         {
-            if ($extrarootdir)
+            if (($installer::globals::iswindowsbuild) && $foundit && $extrarootdir)
             {
                 $destination =~ s,$extrarootdir/,,; # remove it from path
             }