Explicitly indicate no config-site or fatal error if not found

[markcmiller86]

Description

This adjust behavior during CMake. CMake will fatally error if -DVISIT_CONFIG_SITE=NONE is NOT specified and it cannot find a config-site file.

The error will look like...

CMake Error at CMakeLists.txt:687 (message):
  The config-site file,
  /Users/miller86/visit/visit/34rc/src/config-site/scratlantis.cmake, does
  not exist.  Pass -DVISIT_CONFIG_SITE=NONE if no config-site file is needed.

In other words, if you want to configure with -C <someCacheFile> or by specifying a slew of -DVISIT_CMAKE_VARIABLE=VALUE options, you can but you MUST ALSO INDICATE you do not want a config site file with -DVISIT_CONFIG_SITE=NONE (or OFF or NO case does not matter).

This also makes sure we never wind up accidentally caching a value for VISIT_CONFIG_SITE and VISIT_CONFIG_SITE_FILE.