[CMake] Partial linking.

[Óscar Fuentes]

A project is made of a number of libraries. There are cyclic
dependencies. This is solved with partial linking.

Partial linking, in this case, consists on combining a number of object
files to build a single object file (see the -r switch of `ld'), which
is put on ${CMAKE_LIBRARY_OUTPUT_DIRECTORY} and explicitly added to the
link command of executables that depends on the symbols defined on those
object files.

How can I perform partial linking with CMake?

The simplest solution seems to build a regular static library with
add_library, and then apply a custom command that extracts the object
files and performs the partial linking.

-- 
Oscar