[PATCH] cmake: Find xdot or xdot.py

[Matt Arsenault]

  It uses the first one as the basis name of the variables, so trying xdot first will name it HAVE_XDOT/LLVM_PATH_XDOT instead of HAVE_XDOT_PY/LLVM_PATH_XDOT_PY, and it's classier to not include the file extension in the program name.

http://llvm-reviews.chandlerc.com/D715