[CMake] Linking to binary library with a relative path

[GOO Creations]

Hi all,

I have a small problem linking a external library to my project. Here is 
an indication of my directory structure.

root (dir)
  |- external (dir)
      |- external.so (shared library)
  |- mylibs (dir)
      |- <all my libraries>
  |- myprog (executable)

  In the directory "mylibs" all my source code is located. During 
installation all the libraries are also stored in a dir "mylibs" (in the 
install dir).
  I then use Cmake to simply copy "external.so" to the install 
directory. This library is already precompiled (binary). The executable 
is linked to my libraries in the mylibs dir, and one lib in mylibs is 
linked to external.so.

  I use the following statements to link my libs:

ADD_LIBRARY(exter STATIC IMPORTED)
SET_TARGET_PROPERTIES(exter PROPERTIES IMPORTED_LOCATION 
${CMAKE_INSTALL_PREFIX}/external/external.so)
TARGET_LINK_LIBRARIES(mylibx exter)

When I make and install the program, everything works perfectly, but 
mylibx is linked to external.so via a absolute path (eg: 
/home/me/project/install/external/external.so), hence when you rename 
the install folder, the executable will complain that it can't find 
external.so (since the absolute path was changed).
My first thought was to change the following

SET_TARGET_PROPERTIES(exter PROPERTIES IMPORTED_LOCATION 
external/external.so)

Now I have a relative path, but during compilation mylibx throws an 
error that it can't find external.so.
How can I tell Cmake to use the library external.so at a specific path, 
but link mylibx to a relative path?

Thanks for any suggestions.

Christoph