Use the same logic that's used in build.sh/build.cmd to lookup python:
first search for `python3`, then fall back to `python2` and finally to
`python`.
OPTION(CLR_CMAKE_WARNINGS_ARE_ERRORS "Warnings are errors" ON)
# Ensure that python is present
-find_program(PYTHON NAMES python2.7 python2 python)
+find_program(PYTHON NAMES python3 python2 python)
if (PYTHON STREQUAL "PYTHON-NOTFOUND")
message(FATAL_ERROR "PYTHON not found: Please install Python 2.7.9 or later from https://www.python.org/downloads/")
endif()