[build] Fix interface include directories for imported targets

[jacobkahn]

Some versions of CMake (can't figure out exactly which, but some definitely do) don't implicitly add an INTERFACE_INCLUDE_DIRECTORIES property to IMPORTED targets present in generated *Targets.cmake CMake configs.

Specifying INCLUDES DESTINATION include in all install calls for exported targets ensures this property is always present, and fixes cases where linking to imported targets doesn't add the correct include directories.

This PR also fixes an annoying issue where downstream projects would fail to include KenLM headers that transitively included other Kenlm headers (i.e. lm/model.hh) — this was because the installed header dir would be include, but headers would be relative to include/kenlm but placed inside include. Adding include/kenlm to the INSTALL_INTERFACE for all exported targets ensures downstream projects can use their IMPORTED Kenlm targets alone without needing to add hacks to fix include directories.

• That said, this structure isn't great and should probably be fixed one day, cc @kpu.

cc @padentomasello, @andresy, @vineelpratap, @syhw, @tlikhomanenko

[kpu]

I'm thinking of changing all the internal header includes to relative paths. Would that help?

[jacobkahn]

@kpu yep — that would help a lot!