How to add to a library a subdirectory with CMake which has external dependencies?

Usage
1

I’m developing a library called sonovolt, which I want to have different modules that depend on different platforms (boards, like the raspberry pi pico). Here’s the source code. My folder structure looks like this:

/ sonovolt
- /src
-- time.cpp
-- CMakeLists.txt
-- /pico
--- /include/sonovolt/pico.h 
--- /pico.cpp
--- /CMakeLists.txt
- /include
-- /sonovolt
--- /time.h
--- /sonovolt.h
--- /pico.h
- CMakeLists.txt

In the end the result I want is that I can import this library like such, in case it’s the pico:
target_link_libraries(${PROJECT_NAME} pico_stdlib sonovolt sonovolt_pico)
or if the target isn’t the pico (f.ex)
target_link_libraries(${PROJECT_NAME} sonovolt)

In code it should work as:
#include "sonovolt/pico.h"

My problem is that I don’t know how to link platform specific dependencies. The following is the CMakeLists for the pico, which doesn’t build because it can’t find the pico/include/sonovolt/pico.h header or the pico platform specific dependencies. I don’t know how this module should be defined (as INTERFACE) or otherwise.

cmake_minimum_required(VERSION 3.26)

if (NOT TARGET sonovolt_pico)
    include($ENV{PICO_SDK_PATH}/external/pico_sdk_import.cmake)
    set(MAKE_EXPORT_COMPILE_COMMANDS ON)

    # project(sonovolt_pico C CXX ASM)
    pico_sdk_init()

    add_library(sonovolt_pico INTERFACE)
    target_include_directories(sonovolt_pico INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/include/*)
    target_sources(sonovolt_pico 
        PRIVATE 
            ${CMAKE_CURRENT_SOURCE_DIR}/pico.cpp
        PUBLIC
            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
    )
    target_link_libraries(sonovolt_pico INTERFACE sonovolt hardware_pwm)
endif()
2

Globs don’t work here. This should be the $<BUILD_INTERFACE> genex you have later.

And this should list your headers (preferably as a FILE_SET).

I would do this:

target_link_libraries("${PROJECT_NAME}"
  PRIVATE
    sonovolt
    "$<$<PLATFORM_ID:pico>:pico_stdlib>" # can probably be dropped if `sonovolt_pico` publicly depends on this
    "$<$<PLATFORM_ID:pico>:sonovolt_pico>")

You’ll have to change the pico platform ID to whatever platform CMake considers the bit in reality.