This contains header files only, which are also part of
the internal API of libnm-core-impl.
libnm-core-impl is a static library that (among others) implements
libnm-core-public (which is a part of the public API of libnm).
This library gets statically linked into libnm and NetworkManager.
Hence, those components can also access internal (but not private) API of
libnm-core-impl, and this API is in libnm-core-intern.
These headers can thus be included by anybody who statically links with
libnm-core-impl (including libnm-core-impl itself).
The directory should not be added to the include search path, instead
users should explicitly #include "libnm-core-intern/nm-core-internal.h")
There is no source code here and no static library to link against.