Displays a warning when forcing CDAT_BUILD_MODE

If CDAT_BUILD_ESGF is enabled, then CDAT_BUILD_MODE gets forced to
"LEAN". This commit displays a warning when that happens.
Merge pull request #910 from remram44/esgf-mode-warning

Displays a warning when forcing CDAT_BUILD_MODE

looks good to me. thanks @remram44 for the clarifications.