If the variable is NOTFOUND then set it to "". Then at the end after possible list appends. If the variable is not "" then update the target properties.
This gets rid of a warning
The default features are the ones Crow was installed with but can be overridden by setting 'CROW_FEATURES' before the 'find_package' call.
Signed-off-by: Luca Schlecker <luca.schlecker@hotmail.com>