Allow the target to be run automatically

[flagarde]

It would be nice to add an option to add the ALL keyword on the target options to allow it to be run automatically and maybe add an install path relative '${CMAKE_INSTALL_DOCDIR}'.

Something like this pseudo-code?

function(cpm_licenses_create_disclaimer_target TARGET_NAME FILE_NAME PACKAGES)
  cmake_parse_arguments(ARGS "ALL" "PATH" "" ${ARGN})
  set(PACKAGE_SOURCES "")
  foreach(package ${PACKAGES})
    cpm_get_fetch_properties(${package})
    list(APPEND PACKAGE_SOURCES "-D${package}_SOURCE_DIR=\"${${package}_SOURCE_DIR}\"")
  endforeach()

  if(ARGS_ALL)
  add_custom_target(
    ${TARGET_NAME} ALL
    COMMAND
      ${CMAKE_COMMAND} -DPRINT_license_FILE_NAME=${FILE_NAME}
      "-DPRINT_license_PACKAGES=\"${PACKAGES}\"" ${PACKAGE_SOURCES} -P ${CPMlicenses_SCRIPT_PATH}
  )
else()
  add_custom_target(
    ${TARGET_NAME} ALL
    COMMAND
      ${CMAKE_COMMAND} -DPRINT_license_FILE_NAME=${FILE_NAME}
      "-DPRINT_license_PACKAGES=\"${PACKAGES}\"" ${PACKAGE_SOURCES} -P ${CPMlicenses_SCRIPT_PATH}
  )
endif()
INSTALL(FILES ${FILE_NAME} DESTINATION "${CMAKE_INSTALL_DOCDIR}/${ARGS_PATH}")
endfunction()

[TheLartians]

Hey thanks for the issue, I agree that would be a useful addition!
I think in the second branch we should omit the ALL, but feel free to open a PR for a thorough review!