travis: add the possibility of using the txc-dxtn library

travis: add the possibility of using the txc-dxtn library

The txc-dxtn library implements the patented S3 Texture Compression
algorithm.

By default it won't be used but we add the possibility of setting the
USE_TXC_DXTN variable to yes in the travis web UI so it will be
installed and used for the scons tests.

Cc: Eric Anholt <eric@anholt.net>
Cc: Rhys Kidd <rhyskidd@gmail.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
[Emil Velikov: keep the LIB prefix, drop the LD_LIBRARY_PATH, fold URL]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>