Displays a warning when forcing CDAT_BUILD_MODE

Found this commit from some time ago.

If `CDAT_BUILD_ESGF` is enabled, then `CDAT_BUILD_MODE` gets forced to `LEAN`. This commit displays a warning to the user when that happens.
yes but now you are setting it after the check no? so the test for DEFINED CDAT_BUILD_MODE will always be False. no?