[tlbuild] asymptote for tl19

[Karl Berry]

Following Mojca's suggestion, I committed asy 2.47 to the TL source tree
(source/utils/asymptote) so anyone who wishes can try building it.

It compiled ok for me on CentOS7 without any changes to either the
sources or the process (http://tug.org/texlive/build.html#asymptote).

xasy doesn't even come close to actually running for me, as I lack all
the dependencies, starting with "python3" and the Python Qt system,
apparently, but thankfully that didn't prevent building.

For any build problems, the sooner we can report to John
(bowman (at) ualberta (dot) ca), the better. --thanks, karl.