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.
@remram44 @aashish24  shouldn't these two lines be moved back above the warning? I guess it doesn't really matter.