built-in gtest cmake find python order of operations suprise

[cyrush]

In most projects, we use PYTHON_EXECUTABLE to point to python.

In VisIt we use PYTHON_DIR and have logic to derive PYTHON_EXECUTABLE from that.

B/c of this difference - I discovered that gtest will in fact go on to find a python if PYTHON_EXECUTABLE is not set.

gtest's logic lead to VisIt finding the wrong python.

To protect against this, I had to make sure that PYTHON_EXECUTABLE was set before calling BLT.

BLT should defend against this.

[cyrush]

This is what I am using in VisIt:

################################################################
################################################################
# Google Test will look or python
# If PYTHON_EXECUTABLE is not set, it will go on to find
# the wrong python, so this logic protects us when using
# PYTHON_DIR, as VisIt has for some time
################################################################
################################################################
set(CMAKE_DISABLE_FIND_PACKAGE_PythonInterp ON)

message(STATUS "Setting up BLT")
################################################################
# init blt using BLT_SOURCE_DIR
################################################################
include(${BLT_SOURCE_DIR}/SetupBLT.cmake)

# allow us to find python again
set(CMAKE_DISABLE_FIND_PACKAGE_PythonInterp OFF)