Interface library with a generated header file

CMakeLists.txt

cmake_minimum_required(VERSION 3.25)

project(test)

add_executable(hello hello.c)

add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/include/foo.h
  # Ninja always implicitly creates target directories
  COMMAND mkdir -p include/
  COMMAND touch include/foo.h)

add_custom_target(generate_header
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/include/foo.h)

add_library(interface_lib INTERFACE)
# must use add_dependencies to a custom target
# see https://gitlab.kitware.com/cmake/cmake/-/issues/17366
add_dependencies(interface_lib generate_header)
target_include_directories(interface_lib INTERFACE ${CMAKE_CURRENT_BINARY_DIR}/include/)

target_link_libraries(hello interface_lib)