How to create a cmake header-only library that depends on external header files?

I've used an empty _only_for_compiling_the_lib.cpp file as the simplest and fastest workaround, but clearly the above solution is strongly advised.

I simply wasn't aware of INTERFACE keyword.

As stated in the comments, target_include_directories should be given a path to a directory, not to a file.

Moreover, if you want to create a dependency for lib2 on lib1, you should do it through target_link_libraries: a dependency is not only about include directories, but also about compile options, definitions, target properties...

target_sources doesn't work with interface libraries. From this answer, You can use a custom target without commands to associate the sources to a target without impacting the build process (for msvc, QtCreator and other GUI-based tools, this makes the sources accessible through the IDE; AFAIK it's useless for other build tools).

Your cmake may look like this:

add_library(lib1 INTERFACE)
target_sources(lib1 INTERFACE lib1.h)

target_include_directories(lib1 INTERFACE
    "${PROJECT_SOURCE_DIR}/lib1"
)

add_library(lib2 INTERFACE)
if(MSVC)
    add_custom_target(lib2.headers SOURCES lib2.h)
endif()

target_include_directories(lib2 INTERFACE
    "${PROJECT_SOURCE_DIR}/lib2"
)

target_link_libraries(lib2 INTERFACE lib1)

add_executable(project main.cc)
target_link_libraries(project lib2)

Advanced tip: you can specify a different directory in target_include_directories for the build tree and the install tree (see documentation):

target_include_directories(lib1 INTERFACE
    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/lib1>
    $<INSTALL_INTERFACE:${YOUR_INSTALL_DIR}/lib1>
)