[CMake] Why is there no install(DIRECTORY dirs... COMPONENT <component> OPTIONAL)?

[Doug Gregor]

The OPTIONAL argument can be passed to every other variant of the
"install" command, but not the DIRECTORY version. Is there any
particular reason for this limitation, or is it an oversight? I know I
can get around the issue with install(FILES...) and a
file(GLOB_RECURSE), but that's not quite as good.

  - Doug