Changes

core: guard TCP stats on availability of linux/tcp.h

Detect availability of linux/tcp.h at configure time
and build TCP statistics code conditionally based on that.

This replaces platform-specific conditionals with proper feature checks
and avoids build failures on systems lacking Linux TCP headers or types
(e.g. non-Linux or cross-build environments).

When TCP_INFO support is unavailable, the public API remains unchanged,
but all related functions return -ENOTSUP.

No functional changes on systems where linux/tcp.h are available.

Change-Id: Ibcd00f15131b0291f0b10eca51401c518b77cc39

core: always build tun.c and gate TUN support by headers

Always build tun.c and move platform-specific checks into the
implementation.

Guard the TUN-specific code paths based on the availability of
linux/if_tun.h detected at configure time. If TUN support is not
available, osmo_tundev_open() returns -ENOTSUP.

This keeps the public TUN API available while disabling unsupported
functionality in a capability-based way.

Change-Id: I6d1ea1644d12ef59a54cf2f73b9155def58b17a9

core: guard Linux netlink headers by libmnl usage

Only include Linux-specific netlink headers when the libmnl backend
is enabled.

The code paths requiring <linux/if_link.h> and <linux/rtnetlink.h>
are entirely guarded by ENABLE_LIBMNL conditionals, so the headers should
not be included unconditionally.

This avoids pulling in Linux-only headers when the corresponding
backend is not built.

Change-Id: I0b681deb1f1e025e3ea9996f19b1de06a5feb408