[tlbuild] asymptote build

[John Bowman]

I just checked that the latest prerelease of the 1.7 version of cygwin1.dll
no longer requires the empty etc/fstab kludge in C:\Program Files,
which was necessary 6 months ago. We had reports of problems on some
platforms with 1.7 back then, even though it had better thread support, so
eventually we reverted to 1.5 anyway. But once a stable 1.7 release comes
out we will try it again and maybe even set settings.threads=true by default.

On the other hand, set CYGWIN=nodosfilewarning is still required to prevent
the scary warning (I can't understand why the cygwin people think the new
default behaviour is reasonable; I'm sure they will get a lot of complaints
about this when 1.7 becomes the official release).

-- John