seccomp: test for the existence of linux/seccomp.h

Do a compile test for the seccomp header under linux. Yet another
workaround for the raspberry pi.

seccomp mode is not supported by Debian Wheezy:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=675615

Note: seccomp.h is required only for the lookup of the constants defined
in the header.