[Piglit] Build broken since cccc419e2 (python generated tests)

[Kenneth Graunke]

Hi Paul,

cmake dies for me when trying to generate your built-in tests.

On Archlinux, /usr/bin/python and /usr/bin/python3 are Python 3.2, while
Python 2 is /usr/bin/python2.  cmake tries to run your scripts using
plain 'python', which dies quickly.

Mesa ends up checking for the existance of 'python2' first and prefers
that to 'python'.  This works pretty well.  Not sure how to do that in
CMake.

Also, at least some of your new scripts intermix spaces and tabs, which
is frowned upon in 2.x and outright prohibited in 3.x.

--Kenneth