ftjam issue in ArchLinux

I had a lot of trouble getting FT-Jam to do a proper incremental build 
on my ArchLinux system. To make a long story short, it came down to 
incorrect `ar' options. The the default Jambase uses `ar ru`, but it 
needs to be `ar ruU`. The incorrect options cause a warning message: 
"ar: `u' modifier ignored since `D' is the default (see `U')" and makes 
archive members always rebuild even when they are up to date. I guess 
the binutils provided in the Arch repository is configured with 
--enable-deterministic-archives, and that changes the default option of 
the ar command from U to D. I propose changing the default Jambase to 
explicitly include the U option.

On the other hand, I don't see a lot of development activity on Jam and 
the Perforce public depot does not load, so this may be falling on deaf 
ears. In any case, I figured I should say something.

~David