[tlbuild] asymptote search path

[John Bowman]

>     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.

Sure, we can do that.

> 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?

It writes to ~/.asy/history and ~/.asy/history_* so the directory must
be created if it doesn't exist (if that fails, asy will still run but no
history data will be saved).

-- John