Add a dependency not in a subdirectory using CMake - cmake

Question

Let's say there's following directory structure:
root
|
+--projects
| |
| +-test
| |
| +-CMakeFiles.txt
|
+--libs
|
+-testlib
|
+-CMakeFiles.txt
test contains CMakeFiles.txt and testlib also contains CMakeFiles.txt. "test" produces an executable and "testlib" produces a static library.
I want "test" to link with "testlib" without using symlinks and without moving "testlib" library into a subdirectory within "test".
Because "testlib" isn't a subdirectory of "test", I can't do
add_subdirectory("../../libs/testlib")
In test's CMakeFiles.txt - CMake will complain about "testlib" not being in the "test" subdirectory.
Also, because system has several different compilers, I can't simply install "testlib" libraries into some kind of central directory, so I want test to compile a local copy of testlib and link with it (i.e. as if testlib was a subdirectory). I also want the "test" project to automatically rebuild "testlib" if it has been changed.
So, how can I deal with it? I am using CMake 2.8.4 on Windows XP SP3.

Answer 1

You could either provide a top-level CMakeLists.txt in root, or provide a binary directory to the add_subdirectory command; e.g.
add_subdirectory("../../libs/testlib" "${CMAKE_CURRENT_BINARY_DIR}/testlib_build")
This creates a subdirectory called testlib_build in your current build directory which contains the generated project files for testlib, but not the source.
For further info, run
cmake --help-command ADD_SUBDIRECTORY

Answer 2

The only way I see to do this - create CMakeLists.txt in root and put the following code there:
add_subdirectory(projects/test)
add_subdirectory(lib/testlib)
When you have done this, you can do target_link_libraries(test testlib) in test/CMakeLists.txt, and it will be automatically rebuilt if you change something in testlib.