build: check for missing feature files

build: check for missing feature files

linux/dccp.h is unlikely to be installed before 2.6.18 (which was
when headers_install was introduced), and does not exist at all
before 2.6.14. Add a compile-time check to skip compilation of
libxt_dccp in case this was detected.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>