[CMake] how to link with a system library ?

[Steven Samuel Cole]

I would like to link a system library into an executable, GTK in this 
case. What I'm trying to do is

include(FindGTK)

target_link_libraries(myexec $GTK_LIBRARIES)

add_executable (myexec sources)

Result:

CMake Error: Attempt to add link library "GTK_LIBRARIES" to target 
"myexec" which is not built by this project.

What am I doing wrong ?

Kind regards,

Steve