build: Make clang compatibility explicit

build: Make clang compatibility explicit

This change makes explicit that this library only supports clang by
renaming the include directory and only adding it to the include path if
compiling with clang.
A future change may thus explicitly add GCC-compatible headers.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #3