Example of using add_custom_command and add_custom_target together in CMake to handle custom build steps with minimal rebuilding: This example untars library headers for an INTERFACE library target

CMakeLists.txt

set(LIBFOO_TAR_HEADERS
  "${CMAKE_CURRENT_BINARY_DIR}/include/foo/foo.h"
  "${CMAKE_CURRENT_BINARY_DIR}/include/foo/foo_utils.h"
)

add_custom_command(OUTPUT ${LIBFOO_TAR_HEADERS}
  COMMAND ${CMAKE_COMMAND} -E tar xzf "${CMAKE_CURRENT_SOURCE_DIR}/libfoo/foo.tar"
  COMMAND ${CMAKE_COMMAND} -E touch ${LIBFOO_TAR_HEADERS}
  WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/include/foo"
  DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/libfoo/foo.tar"
  COMMENT "Unpacking foo.tar"
  VERBATIM
)

add_custom_target(libfoo_untar DEPENDS ${LIBFOO_TAR_HEADERS})

add_library(foo INTERFACE)
target_include_directories(foo INTERFACE "${CMAKE_CURRENT_BINARY_DIR}/include/foo")
target_link_libraries(foo INTERFACE ${FOO_LIBRARIES})
add_dependencies(foo libfoo_untar)