Produce .lib of header-only library that depends on external resources - cmake

Question

EDIT: I've read up and understood the initial issue was caused by scanning-header-only not having cpp files and thus a lib file not being generated. Edited the question to reflect that extra understanding:
My current project folder structure and relevant CMakeLists content:
leveling
├── CMakeLists.txt: add_subdirectory(deps)
└── deps
├── CMakeLists.txt: add_subdirectory(scanning-header-only)
└── scanning
├── CMakeLists.txt: add_subdirectory(deps)
│ add_library(scanning-header-only file.h)
│ target_include_directories(scanning-header-only PUBLIC ${CMAKE_CURRENT_LIST_DIR}/deps/tinyxml2)
│ target_link_libraries(scanning-header-only PUBLIC tinyxml2)
└── deps
├── CMakeLists.txt: add_subdirectory(tinyxml2)
└── tinyxml2
But a scanning-header-only library file is not being generated, and thus the root project can't target_link_libraries(leveling scanning-header-only) and has had to target_include_directories(leveling ${CMAKE_CURRENT_LIST_DIR}/deps/scanning-header-only/deps/tinyxml2)
Is it possible to target_link_library a header-only library that depends on external resources?
I see that a header-only library without external resource dependency could be add_library(.. INTERFACE), but I'm failing to do so with the dependency on tinyxml2
A dirty workaround is adding and empty cpp file to scanning-header-only so a lib file is generated, but is there a correct way to do this?
Here is minimal example v1: https://www.dropbox.com/s/r1lbajz3xoat1bg/leveling-header-only-test%20v1.zip?dl=0
leveling CMakeLists.txt:
cmake_minimum_required(VERSION 3.8)
set(LEVELING_NAME leveling)
project(${LEVELING_NAME})
#
# To put tinyxml.dll next to the executable, to workaround having to make tinyxml2.dll reachable in PATH
#
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
math(EXPR platform_bits "${CMAKE_SIZEOF_VOID_P} * 8")
set(platform_dir bin/${CMAKE_SYSTEM_NAME}-${platform_bits})
foreach(config DEBUG RELEASE RELWITHDEBINFO MINSIZEREL)
foreach(var
CMAKE_ARCHIVE_OUTPUT_DIRECTORY_${config}
CMAKE_LIBRARY_OUTPUT_DIRECTORY_${config}
CMAKE_RUNTIME_OUTPUT_DIRECTORY_${config}
)
set(${var} "${CMAKE_BINARY_DIR}/${platform_dir}/${config}")
string(TOLOWER "${${var}}" ${var})
endforeach()
endforeach()
#
# ----------------------------------------------------------------------
#
add_subdirectory(deps)
add_executable(${LEVELING_NAME} main.cpp)
target_include_directories(${LEVELING_NAME} PUBLIC
${CMAKE_CURRENT_LIST_DIR}/deps/scanning
)
target_link_libraries(${LEVELING_NAME}
xml-reading
)
set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT ${LEVELING_NAME}) # Set Startup Project in VS. Implemented in CMake v3.6
set_target_properties(${LEVELING_NAME} PROPERTIES VS_DEBUGGER_WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}") # Set Working Directory of project in VS. Implemented in CMake v3.8
scanning CMakeLists.txt
cmake_minimum_required(VERSION 3.8)
set(XML_NAME xml-reading)
project(${XML_NAME})
#
# To put tinyxml.dll next to the executable, to workaround having to make tinyxml2.dll reachable in PATH
#
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
math(EXPR platform_bits "${CMAKE_SIZEOF_VOID_P} * 8")
set(platform_dir bin/${CMAKE_SYSTEM_NAME}-${platform_bits})
foreach(config DEBUG RELEASE RELWITHDEBINFO MINSIZEREL)
foreach(var
CMAKE_ARCHIVE_OUTPUT_DIRECTORY_${config}
CMAKE_LIBRARY_OUTPUT_DIRECTORY_${config}
CMAKE_RUNTIME_OUTPUT_DIRECTORY_${config}
)
set(${var} "${CMAKE_BINARY_DIR}/${platform_dir}/${config}")
string(TOLOWER "${${var}}" ${var})
endforeach()
endforeach()
#
# ----------------------------------------------------------------------
#
add_subdirectory(deps)
add_library(${XML_NAME} INTERFACE CamerasXML.h)
target_include_directories(${XML_NAME} INTERFACE
${CMAKE_CURRENT_LIST_DIR}/deps/tinyxml2
)
target_link_libraries(${XML_NAME}
INTERFACE tinyxml2
)
which yields
CMake Error at deps/scanning/CMakeLists.txt:33 (add_library):
add_library INTERFACE library requires no source arguments.

Answer 1

A .lib is when you create a STATIC (.lib) or SHARED (.lib and .dll) library on Windows. What you want is an INTERFACE library and it generates no files. http://mariobadr.com/creating-a-header-only-library-with-cmake.html has an example. Then you can use the following commands listed here, https://cmake.org/cmake/help/latest/command/add_library.html#interface-libraries, to populate the interface. Notice that it uses INTERFACE not PUBLIC.
target_link_libraries(INTERFACE),
target_link_options(INTERFACE),
target_include_directories(INTERFACE),
target_compile_options(INTERFACE),
target_compile_definitions(INTERFACE), and
target_sources(INTERFACE),
I've never actually used this but I assume it works as documented.

Answer 2

A simple add_library(${XML_NAME} INTERFACE) (not specifying any source files), while having target_include_directories(${XML_NAME} INTERFACE ${CMAKE_CURRENT_LIST_DIR}/deps/tinyxml2) and target_link_libraries(${XML_NAME} INTERFACE tinyxml2) will do the trick.
The tinyxml2 includes are made available to the parent project, and the tinyxml2 library is linked in the parent project.