scons: Always build trace driver

There are so many state trackers and winsys that rely on it for debug
building, and trace can really build anywhere and is thin so there's no
point is making it an option.

Based on  Xavier Chantry <chantry.xavier@gmail.com>'s patch:

"I also removed the trace check in xlib SConscript which is now useless, and
that script seems to deal with the cell driver, so I added cell in the check
for supported drivers."

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