[tlbuild] asymptote build

[John Bowman]

Another little detail: to use the standalone Windows Asymptote binary with
cygwin1.dll from cygwin 1.7 you'll need to create an empty file etc\fstab
in the same directory as asy.exe and cygwin1.dll (unless that glitch has
perhaps already been fixed in the latest 1.7 prerelease).

This is done automatically by our setup file:

http://asymptote.svn.sourceforge.net/viewvc/asymptote/trunk/asymptote/asymptote.nsi