[CMake] Out-of-project link library problem

[Andrew Sayman]

I've been working on linking to some system libraries in a project I'm
building in Linux and I've run into a problem with CMake adding
redundant linker arguments.

The simplest case I came down to is this:
In the Example/Demo/CMakeLists.txt file add the following lines:
target_link_libraries (helloDemo "m" )
target_link_libraries (helloDemo "m" )
target_link_libraries (helloDemo "crypt" "pthread" )

If you turn on verbose makefiles and use the target "Unix Makefiles"
the generated command line has this portion:
-lm -lcrypt -lpthread -lm -lcrypt -lpthread

Essentially, it repeats *all* of the system libraries I specified for
every redundant library that's added.

Changing CMakeLists.txt to something a bit weirder yields the following:
target_link_libraries (helloDemo "m" )
target_link_libraries (helloDemo "m" )
target_link_libraries (helloDemo "crypt" )
target_link_libraries (helloDemo "pthread" )
target_link_libraries (helloDemo "crypt" "pthread" )
-lm -lcrypt -lpthread -lcrypt -lpthread -lm -lcrypt -lpthread -lcrypt -lpthread

The real problem with this is that I use things like FindX11 and
FindPkgCfg to get generated linking libraries for a bunch of modules
on the system. Each of these modules often include some redundant
"base" libraries. For each of those, I end up with this increasingly
expansive list of libraries when I actually only needed them once.

Any ideas? The current workaround I'm using is to generate my own list
of the necessary libraries and remove redundancies before adding it to
the target.
-- 
Andrew Sayman