[tlbuild] asymptote search path

[Karl Berry]

    I would have prefered to consistently use ~/.asy too, but Tomek requested
    the following change. The rationale is that asy writes to this directory
    and ~/.asy might not be writeable (e.g. when using TL from a USB
    stick).

Ah yes.  I forgot that discussion :(.

    We can easily revert this change though (or maybe use some environment
    variable to make it optional?). Let me know what you prefer.

It seems to me that the unusual case of running asy from USB should not
outweigh the advantage of using the common location ~/.asy by default.

How about if we have an envvar, say, ASYHOME.  Unset by default.  The
code effectively does getenv("ASYHOME") || "~/.asy" to find the location.

Then our tl-portable script, and/or users running asy from USB, can set
the envvar, and the rest of the world doesn't need to worry.

One other point: how about if asy does not willy-nilly create the
directory, but only uses it if it's there?  Does it spontaneously put
anything there?

Wdyt?

Thanks,
k