In the case of kindlepdfviewer you do need to set HOST= manually when you run make, it is not "computed" by anything. And you don't need to set TARGET= either.
Yes, you are right --- I just found out that on FC17 64bit one cannot run arm x-tools because of /lib/ld-linux.so.2 ELF interpreter issues. So, using VirtualBox saved me a lot of time and trouble attempting to switch to the latest 64bit OS --- now I know that I should stay with my old good FC12 32bit
But I like the VirtualBox idea... Is it portable? I.e. can I take my vdi files and use them on some other system using any OS (obviously installing VirtualBox software for that OS first)? Or is each vdi image bound to a specific host OS on which it was produced?
And the main achievement of today is that I don't need to dual-boot into Windows XP on my main machine any longer --- I can just install Windows 7 (or XP) inside VirtualBox and run all the Windows-specific programs (Mathematica, Matlab, yes, I know there are Linux versions but they don't really work) in it!