[tlbuild] asymptote build

[John Bowman]

> The 'cygwin1.dll' in bin/win32 seems to belong to 1.5.

The cygwin1.dll library is only included for standalone windows systems;
I agree that it should not be used for an existing cygwin installation.

By the way, the official Asymptote windows setup file associates this asy.bat
file to the Asymptote icon (I don't know if it is useful for TeXLive or not):

@ECHO OFF
set CYGWIN=nodosfilewarning
"C:\Program Files\Asymptote\asy.exe" %*
if %errorlevel% == 0 exit
echo.
PAUSE