You can still using the devil logic of setting options depending on your current Tmux version: see my previous answer.
But since Tmux v1.7, set-option
adds "-q" to silence errors and not print out anything (see changelog).
I recommend to use this feature, it's more readable and easily expandable.
Add this to your ~/.tmux.conf
:
# from v2.1
set -gq mouse on
# before v2.1
set -gq mode-mouse on
set -gq mouse-resize-pane on
set -gq mouse-select-pane on
set -gq mouse-select-window on
Restar tmux or source-file
your new .tmux.conf
Side note: I'm open to remove my old answer if people prefer this one