Fix build on Linux distros that don't support TCP_CONGESTION.

Apparently older kernels don't support TCP_CONGESTION, so we can't
just test for defined(linux) to know if we can use this sockopt or not.
This change unbreaks the build on (notably) CentOS 5.

(cherry picked from commit 0ff7575499d57c2bccd5218e2546b6b0e0426322)
Signed-off-by: Bruce A. Mah <bmah@es.net>