[tlbuild] asymptote search path

[Karl Berry]

    However, instead of getenv I would suggest using system("kpsewhich
    --var-value=ASYMPTOTE_HOME"). 

Ok, sounds good, both the name and using kpsewhich (for the kpse
build, of course), even though it adds one more conditional to John's code.

    if using ~/.asy by default is more important, 

I think it is.  It would certainly surprise all existing asy users for
their existing .asy directory not to be found, or for the new .asy
directory to be placed in an obscure TL-specific location.

Like Manuel, I personally don't think it's necessary to remove ~/.asy on
uninstall -- how could we even tell (in a reasonable way) whether the
user has put stuff there?  They certainly don't want us to blow away
their personally handwritten asy files.

Thanks,
k