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.
