[tlbuild] asymptote for tl19

[Richard Koch]

The new asymptote code compiled for me without errors using the same technique as last year,
on macOS Mojave. Later we'll also compile on Sierra. I ran tests using "make check" and all tests were
passed.

The one interesting warning is that OpenGL is deprecated on the Mac. This is new from last year as
the Mac switches to Metal, which has more direct access to the GPL, but many programs have used
OpenGL in the past and I hope the code will remain in its deprecated state for many years to
come. 

I suspect John Bowman is aware of this deprecation.

Dick Koch