cpp_unvendor()

[romainfrancois]

Some packages (e.g. arrow) go back and forth between vendored and unvendored. An cpp_unvendor() would be nice:

• unlink the inst/include/cpp11 directory
• add LinkingTo directive

[romainfrancois]

Or maybe instead we can have usethis::use_cpp11(vendor = TRUE)

[pachadotdev]

@romainfrancois @DavisVaughan #418