[Cmake] Confused by dependencies

[John Biddiscombe]

If I'm building a library and an executable

SOURCE_FILES(...list here)
ADD_LIBRARY(a library)

then
ADD_EXECUTABLE(an executable).
TARGET_LINK_LIBRARIES(an executable - a library - some other libraries too)

and the executable is dependent upon the library - as well as several 
others. How do I tell it that it is dependent on "a library", but just to 
link with the others regardless. It seems as though no dependency 
information exists when the above commands are carried out.

JB