Can't load enoki through meson

[tbttfox]

The CMakeLists.txt file adds the enoki include directory, but does so at the global level, so there's no base-level target created.
This makes it impossible to load enoki through the cmake interface in meson.
And also, it feels like one of those "good practice" things.

However, I really don't expect any action on this since it would force essentially meaningless updates on all existing downstream packages (plus the source hasn't been updated in 4 years)
It's mostly for people searching this in the future.

And for those future people: CMakeLists.txt would need something like this instead of the global include_directories call.

add_library(enoki INTERFACE)
target_include_directories(enoki INTERFACE include)

Then I think all the other libraries like enoki-cuda would have to link to enoki