removed in linux-v3.10 in commit 1a2c6181c4a1922021b4d7df373bba612c3e5f04

alternatively, we could define it in sys/socket.h since SO* is
reserved there, and tcp.h includes sys/socket.h in extensions mode.

note that SOL_TCP is simply wrong and it's only here for compatibility
with broken applications. the correct argument to pass for setting TCP
socket options is IPPROTO_TCP, which of course has the same value as
SOL_TCP but works everywhere.

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

patch by Isaac Dunham. matched closely (maybe not exact) to glibc's
idea of what _BSD_SOURCE should make visible.