google test finds python undermining ability to find python you want

[cyrush]

Not directly a BLT issue, but BLT should protect against this.

With BLT 0.6.0, which is using a newer google test:

Setting up BLT, which then sets up Google Test, causes system python to be found, undermining options to select a specifc python.

-- PRE BLT: PYTHON_EXECUTABLE /Users/harrison37/Work/github/llnl/conduit/uberenv_libs/spack_view/bin/python3

....

-- Google Test Support is ON
-- Google Mock Support is OFF
-- Found Python: /Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9 (found version "3.9.6") found components: Interpreter 
....

-- POST BLT: PYTHON_EXECUTABLE: /Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9

[cyrush]

Here is the strategy I am using:

################################################################
#! guard against google test's unconditional hunt for python
################################################################
if(DEFINED PYTHON_EXECUTABLE)
    set(_pyexe_guard ${PYTHON_EXECUTABLE})
endif()

set(CMAKE_DISABLE_FIND_PACKAGE_PythonInterp TRUE)
set(CMAKE_DISABLE_FIND_PACKAGE_Python TRUE)
#!##############################################################

################################################################
# init blt using BLT_SOURCE_DIR
################################################################
include(${BLT_SOURCE_DIR}/SetupBLT.cmake)

################################################################
#! guard against google test's unconditional hunt for python
################################################################
set(CMAKE_DISABLE_FIND_PACKAGE_PythonInterp FALSE)
set(CMAKE_DISABLE_FIND_PACKAGE_Python FALSE)

if(DEFINED _pyexe_guard)
    set(PYTHON_EXECUTABLE ${_pyexe_guard})
    unset(_pyexe_guard)
endif()

Setting CMAKE_DISABLE_FIND_PACKAGE_ZZZ isn't sufficient: PYTHON_EXECUTABLE gets cleared when those are set.

[cyrush]

Grumble, even this isn't sufficient.

[cyrush]

resolved in #679