List all subprojects in doxygen main page - cmake

Question

I want to have my doxygen main page to list all my sub projects and refer to their respective README.md file.
In every subproject's CMakeLists.txt (each project is in its own subfolder) I call the function REGISTER_TARGET(my_project_name) which is defined as
macro(REGISTER_TARGET TARGET)
# Storing target name and directory as 2D-array
SET(GLOBAL_TARGETS "${GLOBAL_TARGETS};${TARGET}\;${CMAKE_CURRENT_SOURCE_DIR}" CACHE INTERNAL "source_list")
# doing other things concerning the build
...
endmacro(REGISTER_TARGET)
And in my doxygen's CMakeLists.txt I do
SET(DOXYGEN_GLOBAL_TARGETS "")
foreach(pair ${GLOBAL_TARGETS})
LIST(GET pair 0 local_project_name)
LIST(GET pair 1 local_project_dir)
SET(local_project_readme "${local_project_dir}/README.md")
LIST(APPEND DOXYGEN_FILES ${local_project_readme})
SET(DOXYGEN_GLOBAL_TARGETS "${DOXYGEN_GLOBAL_TARGETS}\n- ${local_project_name}. #ref ${local_project_readme}")
endforeach()
...
CONFIGURE_FILE("${DOXYGEN_MAINFILE_IN}" "${DOXYGEN_MAINFILE}" #ONLY)
And my DOXYGEN_MAINFILE_IN looks like
This is the main page
======
Sub projects
------------
#DOXYGEN_GLOBAL_TARGETS#
This turns out in DOXYGEN_MAINFILE as:
This is the main page
======
Sub projects
------------
- name_of_project1. #ref C:/very/ugly/source/path/name_of_project1/README.md
- name_of_project2. #ref C:/very/ugly/source/path/name_of_project2/README.md
which does not correctly link to this file (in HTML, the whole path is shown and the last path is rendered as hyperlink but refers to an empty file). The HTML for one entry looks similar to
<li>name_of_project1. C:/very/ugly/<a class="el" href="da/ddd/README_8md.html">source/path/name_of_project1/README.md</a></li>
So
a) how can I turn the absolute path to a markdown file in my to a working #ref target (which also displays the first header as link text). Keep in mind, that the above file paths are source-directory so the doc files will reside in a completely different place.
OR
b) which information do i have to serialize in my REGISTER_TARGET CMake macro which can then be turned in such a hyperlink.
I know that it is theoretically possible (OpenCV does it in a quite similar way but I failed to reproduce it)